g10/armor: optimize radix64 to binary conversion

* g10/armor.c (asctobin): Larger look-up table for fast path.
(initialize): Update 'asctobin' initialization.
(radix64_read): Add fast path for radix64 to binary conversion.

This patch adds fast path for radix64 to binary conversion in
armored decryption.

Benchmark results below, tested on Intel Core i7-4790K (turbo off).
Encrypted 2 GiB through pipe to ramfs file using AES128. Decrypt
ramfs file out through pipe to /dev/null.

before patch-set

gpg process

armor: user time pipe transfer rate
encrypt-aead: 13.8 140 MB/s
decrypt-aead: 30.6 68 MB/s
encrypt-cfb: 17.4 114 MB/s
decrypt-cfb: 32.6 64 MB/s

after (decrypt+iobuf+crc+radix64 opt)

gpg process

armor: user time pipe transfer rate
decrypt-aead: 9.8 200 MB/s
decrypt-cfb: 11.9 168 MB/s

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>