Refactor short circuiting identifiers

[Alasdair]

The short-circuiting operators are quite special, but previously they were just identifiers with special names Id "and_bool". Now they have their own constructors in the id_aux type (And_bool/Or_bool), so they can easily be identified in the semantics without any kind of string comparison.

Probably they should have been expressions like E_short_circuit, but that would be a much bigger refactor. Might be worth doing though.

This is the first step in a simplification of identifier handling. Currently the operator ++ could be Id "++" or Operator "++" depending on where it is. Plan is to remove E_app_infix and just use E_app with an attribute indicating if it was desugared from an infix form. Then having Operator x always for operators and Id x for C-style identifiers.

[Alasdair]

Additional points:

• Having and_bool and or_bool as special identifiers does make them work seamlessly with the overloading mechanism, which wouldn't be the case for an expression constructor like E_short_circuit.

• Question of how append_id and prepend_id should work. With this PR they raise an error if used to manipulate and_bool/or_bool. They are both slightly broken with infix operators anyway, but are usually not used to manipulate function identifiers so this isn't an issue in practice.

[github-actions]

Test Results

   15 files     33 suites   0s ⏱️
  957 tests   954 ✅  3 💤 0 ❌
4 695 runs  4 658 ✅ 37 💤 0 ❌

Results for commit fad4f72.

♻️ This comment has been updated with latest results.