Simplify compile_class_not_nested (followup to ECLASS)

[NWilson]

The function compile_class_not_nested is very big and complicated. I reckon we can help it out:

• Always have cranges. For the no-wide-char cases, we can just use a cranges object on the stack (to avoid malloc) and write into that. The wide-char cases would still need to malloc a cranges. But the advantage would be: all the code which follows can avoid having two separate paths.
• We should put the bitmap into the struct cranges. Then, the struct cranges would be a complete description of the characters within the class (that is - all the non-property characters in the class), which would make it easier to reason about. "The characters matched by the class are identified by this bitmap, these cranges/clists, plus the Unicode properties."
• We have code which is split out into separate places. For example, POSIX classes and escapes are handled twice - once when building the cranges, to add in the non-ASCII characters, and again after building the cranges, to add the ASCII characters to the bitmap.

I think we should do build the cranges from the METAs (ideally in one pass, although two passes would be acceptable); then one single pass over the cranges to build the output (without needing to back over the METAs - except possibly just to pick up Unicode property escapes which were completely ignored in the cranges).

Currently, the 16-bit library always, unconditionally, mallocs a cranges object even for super-simple character classes that just match ASCII characters! Using a small stack-alloced cranges as the starting point would be very nice, and we would only grow it to a heap-alloced one if the character class is too big to fit.

Items in the character class should handled in exactly one place: if the item is a character literal or POSIX class then it goes in the cranges, and shouldn't need to be handled elsewhere; if it's a Unicode escape then it's ignored in the cranges and is handled just once later on.

Additionally:

• Consider removing XCL_END

[zherczeg]

The function simplified a lot compared to its original form. It could cache the entire compiled class though. Then no processing during the lengthptr != case, just a memory copy.