Memory optimisation: don't store whole build array

[johnlees]

Simple change: combine build and merge steps so whole array of ska_dict are not stored in memory. On 28 Listeria (3228084, 38Mb skf file)
before:
1 thread 11s, 2.3Gb
8 threads 5s, 3Gb

after:
1 thread 8s, 500Mb
2 threads (max) 6s, 890 Mb

Closes #17 (but more ideas there to further reduce memory if ever needed)

[johnlees]

TODO: fix the clippy lints

[codecov]

Codecov Report

Base: 82.02% // Head: 82.11% // Increases project coverage by +0.09% 🎉

Coverage data is based on head (203d780) compared to base (d777060).
Patch coverage: 87.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   82.02%   82.11%   +0.09%     
==========================================
  Files          15       15              
  Lines        1513     1510       -3     
  Branches      264      265       +1     
==========================================
- Hits         1241     1240       -1     
+ Misses        108      104       -4     
- Partials      164      166       +2     

Impacted Files	Coverage Δ
src/ska_dict/split_kmer.rs	86.13% <ø> (-0.20%)	⬇️
src/ska_ref.rs	85.62% <ø> (-0.18%)	⬇️
src/cli.rs	67.96% <57.14%> (-0.69%)	⬇️
src/lib.rs	72.88% <89.28%> (-0.24%)	⬇️
src/merge_ska_dict.rs	80.53% <90.00%> (+2.97%)	⬆️
src/io_utils.rs	84.90% <100.00%> (+0.29%)	⬆️
src/ska_dict.rs	86.15% <100.00%> (ø)
src/generic_modes.rs	71.42% <0.00%> (-0.88%)	⬇️
... and 4 more

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