Rewrite Cosmopolitan Ar

The build/bootstrap/ar.com program is now tinier. This change reduces
its size from 140kb to 53kb. Nothing was traded away. Cosmopolitan Ar
performance is now 2x better than llvm-ar largely thanks to using the
copy_file_range() system call. This change homebrews a new allocation
API that addresses the shortcomings of the C standard library design.
Using these new balloc() and reballoc() functions I managed to reduce
memory consumption so much that Cosmpolitan Ar should now use roughly
100x fewer bytes of peak resident memory compared to llvm-ar. Correct
behavior with better compatibility has been assured. Binary output is
now pretty much bit-identical to llvm-ar, as of this change. This can
and should be the living proof we need to show that a better world is
possible for software.

libc/elf/getelfstringtable.c

@@ -25,10 +25,16 @@
 /**
  * Returns pointer to elf string table.
  *
- * @param elf points to the start of the executable image
+ * @param elf points to the start of the executable image data
  * @param mapsize is the number of bytes past `elf` we can access
  * @param section_name is usually `".strtab"`, `".dynstr"`, or null
- * @return pointer to double-nul terminated string list or null on error
+ * @return pointer to string table within `elf` image, which should
+ *     normally be a sequence of NUL-terminated strings whose first
+ *     string is the empty string; otherwise NULL is returned, when
+ *     either: (1) `section_name` is not found, (2) it did not have
+ *     the `SHT_STRTAB` section type, (3) the section size was zero
+ *     noting that the ELF spec does consider that legal, or lastly
+ *     (4) an overflow or boundary violation occurred
  */
 char *GetElfStringTable(const Elf64_Ehdr *elf,  //
                         size_t mapsize,         //