Add FASTA-TwoBit converter

[totakke]

Adds FASTA-TwoBit conversion, and test cases also.

As known issue, it is hard to convert a large reference file such as hg38.fa because of memory consumption. The reason is that cljam.io.fasta.reader/sequential-read-string is non-lazy and cljam.io.twobit.reader/read-all-sequences realizes some sequences by chunking. I plan on fixing them on another branch.

[codecov]

Codecov Report

Merging #117 into master will increase coverage by 0.12%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage    85.4%   85.53%   +0.12%     
==========================================
  Files          62       62              
  Lines        4104     4113       +9     
  Branches      401      401              
==========================================
+ Hits         3505     3518      +13     
+ Misses        198      194       -4     
  Partials      401      401

Impacted Files	Coverage Δ
src/cljam/algo/convert.clj	78.57% <100%> (+7.6%)	⬆️
src/cljam/tools/cli.clj	53.66% <66.66%> (-0.36%)	⬇️
src/cljam/io/twobit/reader.clj	79.38% <0%> (+0.76%)	⬆️
src/cljam/io/sequence.clj	100% <0%> (+2.7%)	⬆️
src/cljam/io/fasta/core.clj	94.64% <0%> (+3.57%)	⬆️

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[alumi]

LGTM 👍

I plan on fixing them on another branch.

And thank you for working on the memory issue!