Improve SAM writing speed by ~30%

[jkbonfield]

1. Use the same two bytes at a time approach (seq2nibble) already used
   in CRAM for sequence.

2. Ensure the quality c = s+33 statement doesn't trigger any type
   aliasing rules, permitting SIMD usage.

Benchmarks on 10 million NovaSeq records, reading from uncompressed
BAM (bgzf wrapped):

Read only: ./test/test_view -B /tmp/u.bam
        8993715174      cycles                    #    3.541 GHz
       18985281221      instructions              #    2.11  insn per cycle
       2.540721616 seconds time elapsed

Read + write SAM (develop):
       21910087899      cycles                    #    3.566 GHz
       55089448827      instructions              #    2.51  insn per cycle
       6.144628857 seconds time elapsed

Read + write SAM (this commit):
       18882595700      cycles                    #    3.558 GHz
       42301339830      instructions              #    2.24  insn per cycle
       5.308875782 seconds time elapsed