Convert all HashMap usage to BTreeMap

[jameslittle230]

Fixes #261

❯ while TRUE; cargo run -- build --input local-dev/test-configs/federalist.toml --output test.st 2> /dev/null && sha256sum test.st; end
05ece8e36d4c1abc4362f3969c304288a26500a81df05d37f324ba709581e955  test.st
05ece8e36d4c1abc4362f3969c304288a26500a81df05d37f324ba709581e955  test.st
05ece8e36d4c1abc4362f3969c304288a26500a81df05d37f324ba709581e955  test.st
05ece8e36d4c1abc4362f3969c304288a26500a81df05d37f324ba709581e955  test.st

This is achieved by using a BTreeMap instead of a Hashmap in the Index data structure. This looks like it slows down build times a lot; I'm not sure what effect it has on search times.

Alternatively, I could only convert to a BTreeMap when it comes time to serialize, as described in https://stackoverflow.com/a/42723390 - let's see how metrics look first though.

[codecov[bot]]

Codecov Report

Merging #265 (c5e135a) into master (eeaca67) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #265   +/-   ##
=======================================
  Coverage   72.44%   72.44%           
=======================================
  Files          53       53           
  Lines        2174     2174           
  Branches      104      104           
=======================================
  Hits         1575     1575           
  Misses        598      598           
  Partials        1        1           

Impacted Files	Coverage Δ
stork-lib/src/index_v3/build/fill_stems.rs	100.00% <ø> (ø)
stork-lib/src/index_v3/mod.rs	70.37% <ø> (ø)
stork-lib/src/index_v3/build/fill_containers.rs	91.80% <100.00%> (ø)
stork-lib/src/index_v3/build/mod.rs	98.57% <100.00%> (ø)
stork-lib/src/index_v3/search/mod.rs	95.77% <100.00%> (ø)

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[github-actions[bot]]

Benchmarks

Benchmark	Baseline	Contender	Comparison
build/federalist	202.9417	220.0745	1.08×
federalist.st	1125.456	1125.456	1.0×
search/federalist/liberty	1.9274	1.9648	1.02×
stork.js	21.88	21.88	1.0×
stork.wasm	345.002	356.523	1.03×

Baseline: a7b8bdd69115b812dd9c44909139b146157f6038; Comparison: c5e135aa2f3eeb01501d4568498b36dd989ec7ec