None of this is huge, but it all adds up.

- bam_set1 has been refactored so -O3 is more likely to do unrolling
  and vectorisation.

    // Old          time   inst        cyc
    // gcc -O2      12.36  78936832183 36853852204
    // gcc -O3      12.37  78713347525 36867027825
    // clang13 -O2  12.43  77451926728 37012866717
    // clang13 -O3  12.32  77627221907 36691623424
    // gcc12 -O2    12.43  78895089091 37081260172
    // gcc12 -O3    12.36  78505904437 36829216967

    // New
    // gcc -O2      12.47  78832021505 37200597109 +
    // gcc -O3      12.14  76499369401 36390334338 --
    // clang13 -O2  12.38  76678460761 36920111561 ~
    // clang13 -O3  12.26  76678023071 36548488492 ~
    // gcc12 -O2    12.38  78581694397 36880034181 -
    // gcc12 -O3    12.15  76356625541 36293921439 --

- Improve the MD/NM generation in CRAM decoding.
  With decode_md=1 (default) by decode changed from 12.91s to 12.57s
  With decode_md=0 it's 11.92, so that's 1/3rd of the overhead
  removed.

- Changed the block_resize to resize in slightly smaller chunks and to
  use integer maths.

- Reduce excessive pointer redirection in cram_decode_seq.

  Unsure if this speeds things up much (sometimes it seems to), but it
  provides tidier code too.

Combined before and after on 10 million NovaSeq CRAM (v3.1)

epyc 7543
                   before   after
    gcc(7)  -O2    7.67     7.63   -0.5%
    gcc12   -O2    7.59     7.60   +0.1%
    clang7  -O2    8.12     7.57   -6.8%
    clang13 -O2    8.06     7.54   -6.5%

    gcc(7)  -O3    7.73     7.46   -3.5%
    gcc12   -O3    7.46     7.35   -1.5%
    clang7  -O3    8.08     7.57   -6.3%
    clang13 -O3    7.95     7.66   -3.6%

Xeon Gold 6142
                   before   after
    gcc(7)  -O2    9.74     9.14   -6.2%
    gcc12   -O2    9.43     8.45  -10.4%
    clang7  -O2    9.61     8.64  -10.0%
    clang13 -O2    9.95     8.85  -11.1%

    gcc(7)  -O3    9.51     8.81   -7.4%
    gcc12   -O3    9.15     8.42   -8.0%
    clang7  -O3    9.92     8.72  -12.1%
    clang13 -O3    9.68     8.91   -8.0%

Rewrite bits of the decode_md loop again, fixing bug in previous
commit and finding a perhaps faster route too.

Comparisons with Dev(/D) and this commit (/4) on Revio (re/) and
NovaSeq (nv/) with a variety of compilers and optimisations.  Figures
are cycle counts from perf stat

                     Xeon E5-2660         Xeon Gold 6142
re/D gcc12-O2        85699982958          74752510144
re/4 gcc12-O2        82265084038          71947558666 -3.7/3.7

re/D gcc12-O3        85837077212          74392223354
re/4 gcc12-O3        82024293685          71861154116 -4.4/3.4

re/D clang12-3       85608876213          73934329619
re/4 clang12-3       84390364926          73961392095 -1.4/0

re/D clang12-2       86861787827          74255338533
re/4 clang12-2       83186843797          72421845542 -4.2/2.5; better than O3

nv/D gcc12-O2        36694089398          31444641828
nv/4 gcc12-O2        34949122875          30061074125 -4.8/-4.4

nv/D gcc12-O3        36528573980          30792932748
nv/4 gcc12-O3        35069572111          30066058127 -4.0/2.4

nv/D clang12-3       37906764004          32459168883
nv/4 clang12-3       36344679534          30786987972 -4.1/-5.2

nv/D clang12-2       38443827308          32304948037
nv/4 clang12-2       36361384580          31022553379 -5.4/-4.0

Revised benchmarks on 10 million NovaSeq records, now showing billions
of cycles as more robust than CPU time.

    EPYC 7543
                       before   after
        gcc(7)  -O2    28.6     28.3    -1.0
        gcc12   -O2    28.2     28.3    +0.4
        clang7  -O2    30.2     28.2    -6.6
        clang13 -O2    29.9     28.2    -5.7

        gcc(7)  -O3    28.7     28.2    -1.7
        gcc12   -O3    28.0     27.2    -2.9
        clang7  -O3    30.1     28.3    -6.0
        clang13 -O3    29.7     28.3    -4.7

    Xeon Gold 6142
                       before   after
        gcc(7)  -O2    32.8     30.5    -7.0
        gcc12   -O2    31.8     30.1    -5.3
        clang7  -O2    33.1     29.9    -9.7
        clang13 -O2    34.1     30.8    -9.7

        gcc(7)  -O3    32.7     30.2    -7.6
        gcc12   -O3    31.6     29.1    -7.9
        clang7  -O3    34.3     30.0    -12.5
        clang13 -O3    33.3     30.9    -7.2