Add 'Folder' to replace old types 'Visitor' for most use cases

[kevinbarabash]

This PR also updates the Visitor in escalier_hm to follow the same pattern as the one in escalier_ast.

[codecov[bot]]

Codecov Report

Merging #667 (60c0e19) into main (cd58866) will decrease coverage by 0.35%. The diff coverage is 74.20%.

@@            Coverage Diff             @@
##             main     #667      +/-   ##
==========================================
- Coverage   91.11%   90.76%   -0.35%     
==========================================
  Files         103      104       +1     
  Lines       21658    21817     +159     
==========================================
+ Hits        19734    19803      +69     
- Misses       1924     2014      +90     

Files Changed	Coverage Δ
crates/escalier_hm/src/lib.rs	100.00% <ø> (ø)
crates/escalier_hm/src/visitor.rs	36.93% <40.65%> (-51.55%)	:arrow_down:
crates/escalier_hm/src/folder.rs	77.82% <77.82%> (ø)
crates/escalier_hm/src/util.rs	81.26% <95.45%> (+1.05%)	:arrow_up:
crates/escalier_hm/src/context.rs	100.00% <100.00%> (ø)
crates/escalier_hm/src/infer.rs	91.36% <100.00%> (+0.04%)	:arrow_up:
crates/escalier_hm/src/types.rs	79.95% <100.00%> (ø)

... and 3 files with indirect coverage changes