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 55.24580% with 2986 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.66%. Comparing base (d2224d9) to head (6c9b417).

Files with missing lines	Patch %	Lines
crates/engine_xetex/xetex/xetex_bindings.h	5.96%	0 Missing and 1150 partials ⚠️
crates/engine_xetex/src/c_api/output.rs	74.10%	353 Missing and 46 partials ⚠️
crates/engine_xetex/src/c_api/scaled_math.rs	29.57%	229 Missing and 2 partials ⚠️
crates/engine_xetex/src/c_api/engine/memory.rs	47.77%	129 Missing ⚠️
crates/engine_xetex/src/c_api/dvi.rs	58.42%	73 Missing and 43 partials ⚠️
crates/engine_xetex/xetex/xetex-xetexd.h	1.70%	0 Missing and 115 partials ⚠️
crates/engine_xetex/src/c_api/errors.rs	53.84%	101 Missing and 7 partials ⚠️
crates/engine_xetex/src/c_api/engine.rs	88.67%	77 Missing and 17 partials ⚠️
crates/engine_xetex/xetex/xetex-linebreak.c	57.00%	33 Missing and 59 partials ⚠️
crates/engine_xetex/xetex/xetex-ext.c	15.15%	60 Missing and 24 partials ⚠️
... and 21 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
+ Coverage   31.51%   33.66%   +2.14%     
==========================================
  Files         275      285      +10     
  Lines      180823   186497    +5674     
  Branches   180823   186497    +5674     
==========================================
+ Hits        56994    62790    +5796     
- Misses      44722    45950    +1228     
+ Partials    79107    77757    -1350     

Components	Coverage Δ
tectonic	51.35% <ø> (ø)
bridge_core	37.22% <89.28%> (+0.31%)	⬆️
bridge_flate	38.82% <ø> (ø)
bridge_fontconfig	95.67% <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	40.84% <ø> (ø)
cfg_support	94.84% <ø> (ø)
dep_support	0.00% <ø> (ø)
docmodel	84.87% <ø> (ø)
engine_bibtex	67.46% <ø> (ø)
engine_spx2html	0.00% <ø> (ø)
engine_xdvipdfmx	64.30% <ø> (ø)
engine_xetex	39.67% <55.10%> (+3.93%)	⬆️
errors	47.61% <ø> (ø)
geturl	28.20% <ø> (ø)
io_base	76.11% <ø> (ø)
pdf_io	18.50% <ø> (ø)
status_base	81.92% <ø> (ø)
xdv	55.28% <ø> (ø)
xetex_format	23.31% <ø> (ø)
xetex_layout	58.44% <50.00%> (-1.13%)	⬇️

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