feat: InlineHint metadata

[aborgna-q]

Follow up to #1524. Defines a metadata value for hinting the compiler on which functions to inline.

Part of Quantinuum/hugr#2328.

Note that this definition is rust-only. We still need to add exports for all tket metadata definitions on the python bindings.

Guppy issue/PR (they'll need to be updated):

• feat: (WIP) add metadata and guppy kwarg for inline="always" guppylang#1513
• Inlining annotations guppylang#1054

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.82%. Comparing base (9a25ec8) to head (bb5d1f2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1532      +/-   ##
==========================================
+ Coverage   83.79%   83.82%   +0.02%     
==========================================
  Files         188      188              
  Lines       29032    29036       +4     
  Branches    27928    27932       +4     
==========================================
+ Hits        24326    24338      +12     
+ Misses       3543     3536       -7     
+ Partials     1163     1162       -1     

Flag	Coverage Δ
python	92.32% <ø> (ø)
qis-compiler	91.66% <ø> (ø)
rust	83.48% <100.00%> (+0.03%)	⬆️

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[acl-cqc]

Ok let's combine functionality (not necessarily PRs) here with #1449.
I've updated that to use a similar metadata (there without the Never, with #[non_exhaustive], and I called it InlineAnnotation rather than hint, but at least it's in the same place ;-)).
Noting that #1524 uses inline_acyclic which will mean any function on a cycle will not be inlined at all (cycles are removed from graph rather than merely broken), I think #1449 handles the "Always" bit better, so that can be run first as a separate pass.

Thus, either (a) put this in first, but without the Always option (to be added in 1449); or (b) let's get #1449 in first and then this can add the Never option. I don't really mind which way around, and I think #1449 can be marked ready now (probably for a random reviewer? But you might want to check over the annotation).

WRT this PR:

• I am also unsure about "follow_hints=False" still respecting the "Never" hint; are we sure it shouldn't follow that too? (Otherwise, follow_hints will do nothing when Always is removed. One could say "Always is not a hint" but an annotation, iow it's correct for "follow_hints" to have no effect until we add some InlineAnnotation::XYZHint).
• Would be nice to add to the heuristic here that (non-preserved?) functions with only one call "are always small enough" ;)
• I haven't checked what you're doing here, but I note it's quite tricksy doing inlining respecting PassScope::EntrypointXXX :(