(Fgarbage_collect): Return number of live and free

(Fgarbage_collect): Return number of live and free
strings.

(mark_buffer): Remove code in #if 0.
(gc_sweep): Ditto.
(UNMARK_BALANCE_INTERVALS): Give the macro statement form.
(strings_consed): New variable.
(allocate_string): Set it.
(syms_of_alloc): Add DEFVAR_INT for strings_consed.
(Fmemory_use_counts): Return strings_consed.  Use Flist.

General cleanup in comments etc.  Remove conditional
compilation for `standalone'.

(MARK_STRING, UNMARK_STRING, STRING_MARKED_P):
(GC_STRING_BYTES, GC_STRING_CHARS): New macros.
(DONT_COPY_FLAG): Removed.
(SBLOCK_SIZE, LARGE_STRING_BYTES): New macros.
(struct sdata, struct sblock): New
(struct string_block): Rewritten.
(STRINGS_IN_STRING_BLOCK): New macro.
(oldest_sblock, current_sblock, total_strings, total_free_strings)
(large_sblocks, string_blocks, string_free_list): New variables.
(NEXT_FREE_LISP_STRING, SDATA_OF_STRING, SDATA_SIZE): New macros.
(init_strings): Rewritten.
(allocate_string, allocate_string_data, compact_small_strings)
(free_large_strings, sweep_strings): New functions.
(STRING_BLOCK_SIZE, STRING_BLOCK_OUTSIZE)
(struct string_block_head, current_string_block)
(first_string_block, large_string_blocks, STRING_FULLSIZE)
(STRING_PAD): Removed.
(make_uninit_multibyte_string, make_pure_string): Rewritten.
(Fgarbage_collect): Don't set mark bit in large strings.
(mark_object): Mark strings differently.  Mark symbol names
differently.
(survives_gc_p): Test marked strings differently.
(gc_sweep): Sweep strings differently, unmark strings in
symbol names.
(compact_strings): Removed.