[bibtex] Simplify/reduce global buffer usage

[CraftSpider]

Shiny future would be that the buffers are just vectors, and we largely just pass them around as slices or mutable vectors as relevant.

[codecov]

Codecov Report

❌ Patch coverage is 82.89474% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.17%. Comparing base (de34c24) to head (1a662b2).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/engine_bibtex/src/exec.rs	72.72%	10 Missing and 2 partials ⚠️
crates/engine_bibtex/src/scan.rs	95.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1311   +/-   ##
=======================================
  Coverage   28.17%   28.17%           
=======================================
  Files         266      266           
  Lines      202417   202387   -30     
  Branches   202417   202387   -30     
=======================================
- Hits        57031    57029    -2     
+ Misses      64103    64082   -21     
+ Partials    81283    81276    -7     

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	66.39% <82.89%> (-0.07%)	⬇️
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> (+0.02%)	⬆️
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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