Optimize parser performance

I've gathered basic statistics about the `Symbol` creation and I've got:
```
!| Symbol Name     | Max Children | Usage Rate |
| Whitespace       | 0            | 0.319157   |
| Text             | 0            | 0.236232   |
| SymbolList       | 932          | 0.156875   |
| EndCell          | 0            | 0.118764   |
| Newline          | 0            | 0.049753   |
| CloseBracket     | 0            | 0.027596   |
| CloseBrace       | 0            | 0.021700   |
| OpenBracket      | 0            | 0.013395   |
| Path             | 1            | 0.010051   |
| Variable         | 2            | 0.009809   |
| Colon            | 0            | 0.007823   |
| Comma            | 0            | 0.005611   |
| Bold             | 1            | 0.003850   |
| Delta            | 0            | 0.003060   |
| Table            | 302          | 0.002567   |
| HeaderLine       | 1            | 0.001764   |
| CloseParenthesis | 0            | 0.001534   |
| Alias            | 2            | 0.001520   |
| Italic           | 1            | 0.001288   |
| HashTable        | 13           | 0.001154   |
| CloseLiteral     | 0            | 0.001071   |
| Literal          | 0            | 0.001070   |
| UnorderedList    | 10           | 0.001049   |
| OpenParenthesis  | 0            | 0.000751   |
| Contents         | 5            | 0.000600   |
| Define           | 2            | 0.000525   |
| OpenBrace        | 0            | 0.000396   |
| Comment          | 1            | 0.000268   |
| HorizontalRule   | 0            | 0.000242   |
| CloseCollapsible | 0            | 0.000146   |
| Collapsible      | 2            | 0.000140   |
| AnchorName       | 1            | 0.000057   |
| ClosePreformat   | 0            | 0.000041   |
| WikiWord         | 0            | 0.000035   |
| Include          | 4            | 0.000026   |
| Preformat        | 1            | 0.000021   |
| NoteLine         | 1            | 0.000020   |
| AnchorReference  | 1            | 0.000017   |
| OrderedList      | 12           | 0.000011   |
| Strike           | 0            | 0.000010   |
| Style            | 1            | 0.000001   |
| Link             | 1            | 0.000000   |
| Empty            | 0            | 0.000000   |
```
One sees `Whitespace` and `Text` been about 55% of all `Symbol` instances, and those don't have `children` so allocating new list with default capacity of 10 for every such symbol is unnecessary wasteful.

Further `SymbolList` has reached maximum of 932 children, but getting a statistic for it alone reveals:
```
!| SymbolList         |
| Children | Ratio    |
| 2        | 0.237127 |
| 1        | 0.096052 |
| 3        | 0.055926 |
| 4        | 0.044992 |
| 16       | 0.042085 |
| 158      | 0.036642 |
| 9        | 0.030668 |
| 18       | 0.030595 |
| 20       | 0.025181 |
| 25       | 0.021442 |
| 12       | 0.020876 |
| 14       | 0.019214 |
| 19       | 0.018933 |
| 24       | 0.015310 |
| 5        | 0.015291 |
| 21       | 0.014975 |
| 31       | 0.014845 |
| 6        | 0.014630 |
| 10       | 0.014580 |
| 26       | 0.014450 |
| 56       | 0.014380 |
| 8        | 0.014283 |
| 17       | 0.011847 |
| 32       | 0.011382 |
| 30       | 0.010861 |
| 15       | 0.009933 |
| 7        | 0.008311 |
| 34       | 0.008278 |
| 33       | 0.006184 |
| 13       | 0.006127 |
| 28       | 0.005992 |
| 11       | 0.004867 |
| 35       | 0.004676 |
| 27       | 0.004669 |
| 22       | 0.004530 |
| 248      | 0.004016 |
| 29       | 0.003656 |
| 63       | 0.003534 |
| 37       | 0.003467 |
| 342      | 0.003165 |
| 23       | 0.003006 |
| 185      | 0.002996 |
| 330      | 0.002290 |
| 502      | 0.002032 |
| 47       | 0.001876 |
| 77       | 0.001870 |
| 40       | 0.001712 |
| 79       | 0.001462 |
| 839      | 0.001456 |
| 228      | 0.001450 |
| 48       | 0.001444 |
| 39       | 0.001398 |
| 212      | 0.001349 |
| 41       | 0.001304 |
| 731      | 0.001268 |
| 206      | 0.001191 |
| 681      | 0.001182 |
| 283      | 0.001146 |
| 492      | 0.001138 |
| 440      | 0.001018 |
| 328      | 0.000948 |
| 309      | 0.000894 |
| 306      | 0.000885 |
| 45       | 0.000885 |
| 468      | 0.000812 |
| 140      | 0.000810 |
| 82       | 0.000806 |
| 58       | 0.000805 |
| 197      | 0.000798 |
| 138      | 0.000798 |
| 124      | 0.000789 |
| 36       | 0.000770 |
| 38       | 0.000747 |
| 322      | 0.000745 |
| 44       | 0.000738 |
| 285      | 0.000659 |
| 142      | 0.000657 |
| 279      | 0.000645 |
| 50       | 0.000636 |
| 272      | 0.000629 |
| 49       | 0.000595 |
| 334      | 0.000579 |
| 500      | 0.000578 |
| 250      | 0.000578 |
| 62       | 0.000574 |
| 43       | 0.000572 |
| 932      | 0.000539 |
| 227      | 0.000525 |
| 55       | 0.000509 |
| 281      | 0.000488 |
| 280      | 0.000486 |
| 42       | 0.000486 |
| 46       | 0.000479 |
| 751      | 0.000434 |
| 249      | 0.000432 |
| 186      | 0.000430 |
| 734      | 0.000425 |
| 87       | 0.000403 |
| 139      | 0.000402 |
| 672      | 0.000389 |
| 65       | 0.000376 |
| 159      | 0.000368 |
| 78       | 0.000361 |
| 611      | 0.000353 |
| 189      | 0.000328 |
| 531      | 0.000307 |
| 530      | 0.000307 |
| 527      | 0.000305 |
| 470      | 0.000272 |
| 223      | 0.000258 |
| 109      | 0.000252 |
| 432      | 0.000250 |
| 71       | 0.000246 |
| 379      | 0.000219 |
| 377      | 0.000218 |
| 374      | 0.000216 |
| 53       | 0.000215 |
| 363      | 0.000210 |
| 362      | 0.000209 |
| 360      | 0.000208 |
| 57       | 0.000198 |
| 339      | 0.000196 |
| 333      | 0.000193 |
| 332      | 0.000192 |
| 321      | 0.000186 |
| 319      | 0.000184 |
| 318      | 0.000184 |
| 312      | 0.000180 |
| 305      | 0.000176 |
| 304      | 0.000176 |
| 301      | 0.000174 |
| 299      | 0.000173 |
| 293      | 0.000169 |
| 291      | 0.000168 |
| 72       | 0.000167 |
| 286      | 0.000165 |
| 277      | 0.000160 |
| 269      | 0.000156 |
| 268      | 0.000155 |
| 266      | 0.000154 |
| 66       | 0.000153 |
| 254      | 0.000147 |
| 85       | 0.000147 |
| 252      | 0.000146 |
| 60       | 0.000139 |
| 238      | 0.000138 |
| 232      | 0.000134 |
| 229      | 0.000132 |
| 76       | 0.000132 |
| 111      | 0.000128 |
| 74       | 0.000128 |
| 220      | 0.000127 |
| 219      | 0.000127 |
| 216      | 0.000125 |
| 54       | 0.000125 |
| 213      | 0.000123 |
| 101      | 0.000117 |
| 201      | 0.000116 |
| 193      | 0.000112 |
| 192      | 0.000111 |
| 96       | 0.000111 |
| 64       | 0.000111 |
| 187      | 0.000108 |
| 93       | 0.000108 |
| 184      | 0.000106 |
| 181      | 0.000105 |
| 170      | 0.000098 |
| 166      | 0.000096 |
| 162      | 0.000094 |
| 161      | 0.000093 |
| 160      | 0.000093 |
| 157      | 0.000091 |
| 51       | 0.000088 |
| 69       | 0.000080 |
| 68       | 0.000079 |
| 132      | 0.000076 |
| 120      | 0.000069 |
| 117      | 0.000068 |
| 59       | 0.000068 |
| 113      | 0.000065 |
| 110      | 0.000064 |
| 107      | 0.000062 |
| 105      | 0.000061 |
| 104      | 0.000060 |
| 52       | 0.000060 |
| 98       | 0.000057 |
| 97       | 0.000056 |
| 92       | 0.000053 |
| 81       | 0.000047 |
| 80       | 0.000046 |
| 73       | 0.000042 |
| 70       | 0.000040 |
| 61       | 0.000035 |
| 0        | 0.000000 |
```
So about 33% are instances which have maximum of 1 or 2 children.

The given changes try to address the above findings and few more details.