Prefer inline functions in character.h

Prefer inline functions in character.h

In character.h, replace macros with inline functions or enums
when this is easy.  This improves maintainability and
on my platform (Fedora 31 x86-64, gcc -O2) improved CPU
performance very slightly (0.3%) on ‘make compile-always’.
* src/buffer.h (SANE_TAB_WIDTH, CHARACTER_WIDTH):
Move here from character.h, and make them inline functions.
Tune CHARACTER_WIDTH so that ASCII_CHAR_WIDTH is no longer needed.
(sanitize_tab_width, sanitize_char_width):
Move here from character.h.
* src/character.h (MAX_CHAR, MAX_UNICODE_CHAR, MAX_1_BYTE_CHAR)
(MAX_2_BYTE_CHAR, MAX_3_BYTE_CHAR, MAX_4_BYTE_CHAR)
(MAX_5_BYTE_CHAR, MIN_MULTIBYTE_LEADING_CODE)
(MAX_MULTIBYTE_LEADING_CODE, MAX_MULTIBYTE_LENGTH):
Now enum constants instead of macros.
* src/character.h (CHAR_BYTES): Redo to avoid conditional branches.
(CHAR_BYTE8_P, BYTE8_TO_CHAR, UNIBYTE_TO_CHAR, CHAR_TO_BYTE8)
(CHAR_TO_BYTE_SAFE, CHAR_BYTE8_HEAD_P, CHARACTERP)
(CHECK_CHARACTER, CHECK_CHARACTER_CAR, CHECK_CHARACTER_CDR)
(CHAR_PRINTABLE_P, CHAR_BYTES, CHAR_LEADING_CODE, BYTE8_STRING)
(LEADING_CODE_P, TRAILING_CODE_P, CHAR_HEAD_P)
(BYTES_BY_CHAR_HEAD):
Now inline functions instead of macros.
(ASCII_CHAR_WIDTH): Remove; no longer used.
* src/conf_post.h (ATTRIBUTE_PURE): New macro.
* src/lisp.h (char_table_ref): Use it, for better inlining.
* src/fns.c (base64_decode_1): Add now-necessary casts.