Some more tweaks of the libbacktrace zstd decompressor to make
decompressing slightly faster: unpack all the literal data into the
output buffer, rather than using scratch space.  Bootstrapped and ran
libbacktrace tests on x86_64-pc-linux-gnu.  Committed to mainline.

Ian

* elf.c (elf_fetch_backward_init): New static function.
(ZSTD_TABLE_SIZE): Use huffman scratch space size rather than
literal size.
(ZSTD_TABLE_WORK_LIT_SIZE): Don't define.
(elf_zstd_read_huff): Use elf_fetch_backward_init.
(elf_zstd_read_literals): New static function.
(ZSTD_LIT_RAW, ZSTD_LIT_RLE, ZSTD_LIT_HUFF): Don't define.
(struct elf_zstd_literals): Don't define.
(elf_zstd_literal_output): Remove static function.
(elf_zstd_decompress): Use elf_fetch_backward_init and
elf_zstd_read_literals.  Rewrite literal copying.<