Port engine_xetex to Rust

[CraftSpider]

This is the big one - begin converting the engine into Rust.

CI clippy and Windows vcpkg are consistently failing. I'm leaving off fixing those till the end, as long as every other check is passing. vcpkg is a weird one in particular, it's related to a library on Windows not getting linked but that apparently works without these (non-build) changes. So I'm hoping removing most of the C code will just make it fix itself eventually.

[codecov]

Codecov Report

❌ Patch coverage is 68.05737% with 980 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.07%. Comparing base (e8dd681) to head (34e0b40).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/engine_xetex/src/c_api/scaled_math.rs	31.71%	209 Missing and 2 partials ⚠️
crates/engine_xetex/src/c_api/output.rs	80.10%	131 Missing and 22 partials ⚠️
crates/engine_xetex/xetex/xetex-xetexd.h	0.93%	0 Missing and 106 partials ⚠️
crates/engine_xetex/xetex/xetex-ini.c	74.01%	0 Missing and 92 partials ⚠️
crates/engine_xetex/src/c_api/engine.rs	88.37%	67 Missing and 4 partials ⚠️
crates/engine_xetex/xetex/xetex-linebreak.c	57.14%	33 Missing and 36 partials ⚠️
crates/engine_xetex/xetex/xetex-shipout.c	52.27%	3 Missing and 60 partials ⚠️
crates/engine_xetex/src/c_api/engine/memory.rs	32.50%	54 Missing ⚠️
crates/engine_xetex/xetex/xetex-texmfmp.c	30.00%	10 Missing and 18 partials ⚠️
crates/engine_xetex/src/c_api/pool.rs	82.46%	27 Missing ⚠️
... and 13 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
+ Coverage   28.85%   29.07%   +0.21%     
==========================================
  Files         273      281       +8     
  Lines      203491   203685     +194     
  Branches   203491   203685     +194     
==========================================
+ Hits        58724    59226     +502     
- Misses      64143    64357     +214     
+ Partials    80624    80102     -522     

Components	Coverage Δ
tectonic	50.89% <ø> (ø)
bridge_core	34.44% <88.46%> (-0.54%)	⬇️
bridge_flate	38.82% <ø> (ø)
bridge_fontconfig	96.17% <ø> (ø)
bridge_freetype2	92.70% <ø> (ø)
bridge_graphite2	42.47% <ø> (ø)
bridge_harfbuzz	82.83% <ø> (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	66.45% <ø> (ø)
engine_spx2html	0.00% <ø> (ø)
engine_xdvipdfmx	55.58% <ø> (ø)
engine_xetex	35.76% <67.88%> (+0.48%)	⬆️
errors	47.61% <ø> (ø)
geturl	27.16% <ø> (ø)
io_base	76.04% <ø> (ø)
pdf_io	14.29% <ø> (-0.03%)	⬇️
status_base	71.76% <ø> (ø)
xdv	55.21% <ø> (ø)
xetex_format	22.80% <ø> (ø)
xetex_layout	58.62% <ø> (ø)

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