[EXP] use codon degeneracy to properly translate edge amino acids if possible

[bluegenes]

Addresses #659

• for 2-amino acid long codons (e.g. k-mers at the end of translated sequence), use N as last base to enable translating codons if possible, e.g. https://github.com/dib-lab/sourmash/blob/eb2b210d40b1441a1467ec507e920339e0bfe437/src/core/src/encodings.rs#L96-L101

This functionality already existed in translate_codon, here I modified the to_aa function to allow 2-length codons to be passed into translate_codon.

[codecov]

Codecov Report

Merging #1579 (d85cbfc) into latest (6b5806c) will increase coverage by 8.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1579      +/-   ##
==========================================
+ Coverage   81.10%   89.28%   +8.18%     
==========================================
  Files         102       75      -27     
  Lines       10334     6626    -3708     
  Branches     1171     1171              
==========================================
- Hits         8381     5916    -2465     
+ Misses       1748      505    -1243     
  Partials      205      205              

Flag	Coverage Δ
python	89.28% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/core/src/ffi/utils.rs
src/core/src/ffi/nodegraph.rs
src/core/src/lib.rs
src/core/src/signature.rs
src/core/src/ffi/minhash.rs
src/core/tests/minhash.rs
src/core/src/cmd.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/wasm.rs
src/core/src/ffi/signature.rs
... and 16 more

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