Add an (inferred) .scheme field on TypeRefs

[kevinbarabash]

This will allow us to use variables whose type is private. There's still some kinks to work out with type params that reference themselves.

[codecov]

Codecov Report

Merging #720 (e25d381) into main (3d52571) will increase coverage by 0.51%.
The diff coverage is 92.43%.

@@            Coverage Diff             @@
##             main     #720      +/-   ##
==========================================
+ Coverage   84.40%   84.92%   +0.51%     
==========================================
  Files          62       62              
  Lines       12706    12713       +7     
==========================================
+ Hits        10725    10797      +72     
+ Misses       1981     1916      -65     

Files	Coverage Δ
crates/escalier_ast/src/class.rs	87.50% <ø> (ø)
crates/escalier_ast/src/visitor.rs	58.03% <ø> (+2.15%)	⬆️
crates/escalier_hm/src/context.rs	97.87% <100.00%> (+0.03%)	⬆️
crates/escalier_hm/src/infer.rs	94.64% <100.00%> (+0.09%)	⬆️
crates/escalier_hm/src/infer_class.rs	79.46% <100.00%> (+7.23%)	⬆️
crates/escalier_hm/src/types.rs	86.13% <100.00%> (+0.10%)	⬆️
crates/escalier_hm/src/unify.rs	85.91% <100.00%> (+0.06%)	⬆️
crates/escalier_hm/src/visitor.rs	37.59% <100.00%> (ø)
crates/escalier_interop/src/parse.rs	91.42% <100.00%> (+0.14%)	⬆️
crates/escalier_interop/src/util.rs	85.29% <100.00%> (+0.60%)	⬆️
... and 4 more