Add internal function & document. More comments, new allocation structure.

Add a function "ualphabet_rejected" returning the number of array
entries which the Unicode alphabet rejected.

Change the "fake unicode" to use a string within "tf" called
"fake_unicode". This saves repeated calloc/free cycles for "b", the
non-Unicode string compared with a Unicode string in "tf".

Document internal functions under a new heading, may be useful for
benchmarking etc.