Prefer functions to macros in buffer.h

Prefer functions to macros in buffer.h

In buffer.h, prefer inline functions to function-like macros
when either will do.  This helps avoid confusion about how
many times an arg is evaluated.  On my platform, this patch
improves performance of ‘make compile-always’ by 5.7%.
Also, prefer enum constants to object-like macros
when either will do.
* src/buffer.h (BEG, BEG_BYTE, GAP_BYTES_DFL, GAP_BYTES_MIN)
(MAX_PER_BUFFER_VARS, NONEXISTENT_MODTIME_NSECS)
(UNKNOWN_MODTIME_NSECS, BUFFER_LISP_SIZE, BUFFER_REST_SIZE):
Now enum constants, instead of macros.
(BUFFER_CEILING_OF, BUFFER_FLOOR_OF, BUF_BEG, BUF_BEG_BYTE)
(BUF_BEGV, BUF_BEGV_BYTE, BUF_PT, BUF_PT_BYTE, BUF_ZV)
(BUF_ZV_BYTE, BUF_GPT_ADDR, BUF_Z_ADDR, BUF_GAP_END_ADDR)
(BUF_COMPUTE_UNCHANGED, SET_PT, TEMP_SET_PT, SET_PT_BOTH)
(TEMP_SET_PT_BOTH, BUF_TEMP_SET_PT, SET_BUF_BEGV, SET_BUF_ZV)
(SET_BUF_BEGV_BOTH, SET_BUF_ZV_BOTH, SET_BUF_PT_BOTH)
(BYTE_POS_ADDR, CHAR_POS_ADDR, CHAR_TO_BYTE, BYTE_TO_CHAR)
(PTR_BYTE_POS, FETCH_CHAR, FETCH_CHAR_AS_MULTIBYTE)
(BUF_BYTE_ADDRESS, BUF_CHAR_ADDRESS, BUF_PTR_BYTE_POS)
(BUF_FETCH_CHAR, BUF_FETCH_CHAR_AS_MULTIBYTE, BUF_FETCH_BYTE)
(BUFFER_PVEC_INIT, BUFFER_LIVE_P, BUFFER_HIDDEN_P)
(BUFFER_CHECK_INDIRECTION, OVERLAY_POSITION, PER_BUFFER_VALUE_P)
(SET_PER_BUFFER_VALUE_P, PER_BUFFER_IDX):
Now inline functions instead of macros.