refactor: introduce AsciiChar wrapper over u8

[ivan-aksamentov]

Followup of #301

The AsciiChar wrapper type allows:

• better debugging, as we can control how AsciiChar is printed, as opposed to the integer-like u8 type - this is mostly a dev convenience thing - for debug-printing sequences, various maps etc., but it boosts development significantly
• provide valid outputs wherever we need to display or write to file any characters - this is actually needed for correctness

I measured no significant slowdown:

Command:

$ cargo -q build --release --target-dir=/workdir/.build/docker --bin=treetime
$ hyperfine --warmup 1 --show-output '/workdir/.build/docker/release/treetime ancestral --method-anc=parsimony --dense=false --tree=data/mpox/clade-ii/500/tree.nwk --outdir=tmp/smoke-tests/ancestral/marginal/mpox/clade-ii/500 data/mpox/clade-ii/500/aln.fasta.xz'

Before (branch: rust, commit 67ba54a):

  Time (mean ± σ):     655.4 ms ±  25.7 ms    [User: 2258.9 ms, System: 210.1 ms]
  Range (min … max):   635.3 ms … 717.6 ms    10 runs

After:

  Time (mean ± σ):     650.3 ms ±  26.9 ms    [User: 2231.2 ms, System: 218.7 ms]
  Range (min … max):   630.9 ms … 716.2 ms    10 runs