Expand selectors as hex-encoded literals in metadata

[Robbepop]

Extracted from work in done in #665.

Using the utilities provided in this PR we can also use hex-encoded literals elsewhere in our macro expansion.

[codecov-commenter]

Codecov Report

Merging #927 (0ef98cc) into master (4155ebe) will increase coverage by 0.07%.
The diff coverage is 95.45%.

❗ Current head 0ef98cc differs from pull request most recent head f6a900a. Consider uploading reports for the commit f6a900a to get more accurate results

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
+ Coverage   84.24%   84.31%   +0.07%     
==========================================
  Files         172      173       +1     
  Lines        7945     7958      +13     
==========================================
+ Hits         6693     6710      +17     
+ Misses       1252     1248       -4     

Impacted Files	Coverage Δ
crates/lang/ir/src/ir/selector.rs	85.00% <87.50%> (+1.66%)	⬆️
crates/lang/codegen/src/generator/metadata.rs	97.56% <100.00%> (-0.04%)	⬇️
crates/lang/ir/src/literal.rs	100.00% <100.00%> (ø)
crates/allocator/src/bump.rs	100.00% <0.00%> (+20.00%)	⬆️

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[HCastano]

Question about the pattern used, but code/tests look good

[HCastano]

Can you describe how this pattern works?

[Robbepop]

The Sealed type is inaccessible from the outside therefore one cannot provide it (in safe Rust) as parameter for the hidden method and therefore we protect to call it from outside the default trait methods using the Rust type system.