[experimental] guess how much memory will be needed and adjust chunk size

The guessed memory is adequate to the total file size of sources given. I don't think it's a good idea but the results are rather good.

Benchmarked using `avgtime -d -r 10 dmd std/algorithm -unittest -o-`.

Default DMD:
```
Total time (ms): 60364.4
Repetitions    : 10
Sample mode    : 6190 (1 ocurrences)
Median time    : 6057.17
Avg time       : 6036.44
Std dev.       : 127.367
Minimum        : 5769.23
Maximum        : 6196.66
95% conf.int.  : [5786.8, 6286.07]  e = 249.634
99% conf.int.  : [5708.36, 6364.51]  e = 328.075
EstimatedAvg95%: [5957.5, 6115.38]  e = 78.9412
```
Guessing DMD:
```
Total time (ms): 51912.9
Repetitions    : 10
Sample mode    : 5170 (2 ocurrences)
Median time    : 5167.91
Avg time       : 5191.29
Std dev.       : 139.772
Minimum        : 5030.62
Maximum        : 5520.05
95% conf.int.  : [4917.34, 5465.23]  e = 273.947
99% conf.int.  : [4831.26, 5551.31]  e = 360.028
EstimatedAvg95%: [5104.66, 5277.92]  e = 86.6297
EstimatedAvg99%: [5077.44, 5305.14]  e = 113.851
```

Even though the guess/needed ratio is 0.07 so 7% of the memory needed is preallocated, there is a ~16% speedup. The allocated/needed ratio is at ~1.0054 which is quite good. There are some printfs you can use to adjust the magic numbers or to try different source files.