Implement pattern matching with literals and guards

[b-studios]

This PR implements proposal #383. A working example can be found here:

• https://effekt-lang.org/examples/matching.html

Notes about the implementation can be found at the top of

• PatternMatchingCompiler.scala
• ExhaustivityChecker.scala

The implementation of the compiler was rather modest; exhaustivity checking was surprisingly tricky since it is difficult to maintain the necessary information (like types to check exhaustivity of nested scrutinees).

Both implementations (compiler and checker) are structure to

1. preprocess clauses into an internal representation (transformation requires Context)
2. perform the actual work (transformation does NOT require Context and can thus be tested more easily)

Note: for now, I decided that Or-Patterns are not worth it.

---

Implemented in frontend

• pattern guards
• boolean guards
  - [x] or patterns (properly, which requires significant changes to always bind the SAME symbol)
• literal patterns
• guards on if and while
• exhaustivity check

pattern compiler

• guards
  - [ ] or patterns
• literal patterns
• heuristic
• compilation of if and while
  - [ ] fuse subsequent predicates using && but also make && by-name in its arguments.

translation

• default clause in matches
• non trivial guards in while and if

[b-studios]

Reminder to future me: Hey Jonathan, nice that you have some time for programming again; if you really want to add Or-Patterns again, start by reverting d4452a3.

[b-studios]

To fix in the future: Using Map[ValueVar, Pattern] as a semantic domain for patterns in the compiler is wrong. It should be List[(ValueVar, Pattern)] since we might have multiple matches on the same scrutinee.