Coding system support cleanup and minor refactoring.

Coding system support cleanup and minor refactoring.
* coding.h (enum coding_result_code): Remove
CODING_RESULT_INCONSISTENT_EOL and CODING_RESULT_INSUFFICIENT_MEM.
(toplevel): Remove unused CODING_MODE_INHIBIT_INCONSISTENT_EOL.
(CODING_MODE_LAST_BLOCK, CODING_MODE_SELECTIVE_DISPLAY)
(CODING_MODE_DIRECTION, CODING_MODE_FIXED_DESTINATION)
(CODING_MODE_SAFE_ENCODING): Rearrange bit values.
(decode_coding_region, encode_coding_region, decode_coding_string):
Remove unused compatibility macros.
* coding.c (Qinconsistent_eol, Qinsufficient_memory): Remove.
(record_conversion_result): Adjust user.
(syms_of_coding): Likewise.
(ALLOC_CONVERSION_WORK_AREA): Use SAFE_ALLOCA.
(decode_coding, encode_coding): Add USE_SAFE_ALLOCA and SAFE_FREE.
(decode_coding_object): Simplify since xrealloc never returns NULL.
Add eassert.