Sort-dependent expansion for Modules

[disconcision]

This implemented sort-dependent expansion. Multi-delimiter forms now only expand when inserted in an appropriate sort context. This is a slightly subtle notion; it currently means that the nearest leftward non-secondary piece has a nib of the appropriate sort. This has a complex but well-tested special case around case rules.

The new functionality is demonstrated below by creating four new forms of pattern sort. They are patterns because I didn't want to bother creating a new sort, but are intended to be representative of what we'd need to implement module syntax. The let and type forms below reuse Exp keywords, but now in a pattern context they expand different. ; is an infix operator below, not a trailing delimiter. AFAICT this suffices to implement module syntax; see appendix below.

Appendix:

Example of module syntax which the above enables:

	let M : {
	  type T = Int;
	  let a: T
	} = {
	  type T = Int;
	  let a = 4;
	  let b = a
	}

Required for modules: 2 new sorts: Mod and Sig and 8 new forms:

Typ:= ...
  | "{" Sig "}"
Exp ::= ... 
  | "{" Mod "}"
Mod ::=
  | Mod ";" Mod
  | "type" TPat "=" Typ
  | "let" Pat "=" Exp
Sig ::=
  | Sig ";" Sig
  | "type" TPat "=" Typ
  | "let" Pat

Remaining issues:

• If you copy an expanded form to a new sort context, or deconstruct its parent sort context, it will not remold or disintegrate; i.e. to turn a module let into a regular let or vice versa you'll need to delete all delimiters. Getting this to work is a larger change; I'm not sure on a good approach. Copy-paste could be hacked to work somewhat straightforwardly though (just skip the clipboard cache for segments containing forms with multiple expansions)
• Probably want an error for reserved words (expanding delimiters used out-of-sort-context)
• Much more functional testing needed; implementing modules should handle this though.

[codecov]

Codecov Report

❌ Patch coverage is 94.20290% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.72%. Comparing base (bb290a6) to head (745ee4d).

Files with missing lines	Patch %	Lines
src/haz3lcore/lang/Form.re	94.44%	2 Missing ⚠️
src/haz3lcore/TyDi/TyDiForms.re	87.50%	1 Missing ⚠️
src/haz3lcore/zipper/action/Insert.re	85.71%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           probe-III    #2075      +/-   ##
=============================================
+ Coverage      47.55%   47.72%   +0.17%     
=============================================
  Files            229      229              
  Lines          24937    24955      +18     
=============================================
+ Hits           11860    11911      +51     
+ Misses         13077    13044      -33     

Files with missing lines	Coverage Δ
src/haz3lcore/tiles/Segment.re	66.17% <100.00%> (ø)
src/haz3lcore/zipper/Ancestor.re	75.00% <ø> (+26.61%)	⬆️
src/haz3lcore/zipper/Relatives.re	78.78% <100.00%> (+2.38%)	⬆️
src/haz3lcore/TyDi/TyDiForms.re	53.16% <87.50%> (-0.69%)	⬇️
src/haz3lcore/zipper/action/Insert.re	91.40% <85.71%> (+0.06%)	⬆️
src/haz3lcore/lang/Form.re	93.08% <94.44%> (+0.49%)	⬆️

... and 13 files with indirect coverage changes

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