[bibtex] Refactor string pool

[CraftSpider]

Encapsulates and converts to idiomatic Rust the StringPool.

[codecov]

Codecov Report

❌ Patch coverage is 74.79339% with 183 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.16%. Comparing base (14aefdc) to head (3afa239).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/engine_bibtex/src/lib.rs	62.68%	19 Missing and 56 partials ⚠️
crates/engine_bibtex/src/exec.rs	66.46%	49 Missing and 7 partials ⚠️
crates/engine_bibtex/src/pool.rs	85.36%	14 Missing and 4 partials ⚠️
crates/engine_bibtex/src/bibs.rs	47.05%	9 Missing ⚠️
crates/engine_bibtex/src/auxi.rs	25.00%	1 Missing and 5 partials ⚠️
crates/engine_bibtex/src/bst.rs	89.36%	4 Missing and 1 partial ⚠️
crates/engine_bibtex/src/cite.rs	66.66%	4 Missing ⚠️
crates/engine_bibtex/src/hash.rs	96.22%	4 Missing ⚠️
crates/engine_bibtex/src/global.rs	25.00%	3 Missing ⚠️
crates/engine_bibtex/src/scan.rs	91.30%	0 Missing and 2 partials ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1308      +/-   ##
==========================================
+ Coverage   28.12%   28.16%   +0.03%     
==========================================
  Files         266      267       +1     
  Lines      202442   202507      +65     
  Branches   202442   202507      +65     
==========================================
+ Hits        56936    57027      +91     
+ Misses      64147    64141       -6     
+ Partials    81359    81339      -20     

Components	Coverage Δ
tectonic	50.89% <ø> (ø)
bridge_core	34.73% <ø> (ø)
bridge_flate	38.82% <ø> (ø)
bridge_fontconfig	83.06% <ø> (ø)
bridge_freetype2	92.00% <ø> (ø)
bridge_graphite2	∅ <ø> (∅)
bridge_harfbuzz	∅ <ø> (∅)
bridge_icu	∅ <ø> (∅)
bridge_png	100.00% <ø> (ø)
bundles	34.78% <ø> (ø)
cfg_support	94.84% <ø> (ø)
dep_support	0.00% <ø> (ø)
docmodel	84.09% <ø> (ø)
engine_bibtex	65.11% <74.79%> (+0.78%)	⬆️
engine_spx2html	0.00% <ø> (ø)
engine_xdvipdfmx	55.58% <ø> (ø)
engine_xetex	35.28% <ø> (ø)
errors	47.61% <ø> (ø)
geturl	27.16% <ø> (ø)
io_base	76.04% <ø> (ø)
pdf_io	14.31% <ø> (-0.01%)	⬇️
status_base	71.76% <ø> (ø)
xdv	0.00% <ø> (ø)
xetex_format	22.80% <ø> (ø)
xetex_layout	42.81% <ø> (ø)

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[CraftSpider]

This passes tectonic-on-arXiv, so no changes. There's a little fib in the logs now - they claim to have a max strings, but this change uncaps the StringPool. It now grows more dynamically, meaning max strings is no longer meaningful. But the goal is byte-for-byte compat, so for now, we keep that log the same.

(I don't think allowing more strings breaks compat, as it would allow strictly more things to work. Making something not an error isn't breaking, as no one could have been writing something that worked with it before anyways.)