Add _unused names

[jmackie]

Currently, any unused value binders raise a warning. For example:

ditto/crates/ditto-checker/golden-tests/warnings/unused_function_binder.warnings

Lines 2 to 9 in 53a0eb4

	⚠ unused function binder
	╭─[golden:1:1]
	1 │ module Test exports (..);
	2 │
	3 │ always_five = (ignore) -> 5;
	· ───┬──
	· ╰── this isn't used
	╰────

ditto/crates/ditto-checker/golden-tests/warnings/unused_pattern_binder.warnings

Lines 2 to 11 in 53a0eb4

	⚠ unused patter binder
	╭─[golden:4:1]
	4 │
	5 │ not_used = (o: Option(a)): Float ->
	6 │ match o with
	7 │ | Some(a) -> 5.0
	· ┬
	· ╰── this isn't used
	8 │ | None -> 5.0;
	╰────

ditto/crates/ditto-checker/golden-tests/warnings/unused_effect_binder.warnings

Lines 2 to 12 in 53a0eb4

	⚠ unused effect binder
	╭─[golden:5:1]
	5 │ };
	6 │
	7 │ greet : Effect(Unit) = do {
	8 │ name <- get_name;
	· ──┬─
	· ╰── this isn't used
	9 │ return unit
	10 │ };
	╰────

Clearly there are legitimate reasons to ignore a binding, so there needs to be a way to suppress these warnings. Most languages (and/or linters) that I'm aware of handle this by prefixing unused variables with a _.

However this is generally just a convention, as variables marked as unused (by a leading underscore) can usually still be referenced. And I'm not the only one who would prefer it if this convention was enforced by the compiler:

rust-lang/rust#3423
golang/go#39118

Ditto will enforce this by not treating _unused variables as valid expressions. In this PR this is enforced at the parser level, but that might change in future in the interest of more helpful error messages...

Note that this PR doesn't extend Effect::Bind with unused binders, that will be implemented as part of #46.

[codecov]

Codecov Report

Merging #45 (6b84a25) into main (53a0eb4) will increase coverage by 0.02%.
The diff coverage is 85.10%.

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   77.23%   77.26%   +0.02%     
==========================================
  Files         109      109              
  Lines        6836     6914      +78     
==========================================
+ Hits         5280     5342      +62     
- Misses       1556     1572      +16     

Impacted Files	Coverage Δ
...ates/ditto-checker/src/typechecker/substitution.rs	94.11% <ø> (ø)
crates/ditto-cst/src/name.rs	100.00% <ø> (ø)
crates/ditto-fmt/src/has_comments.rs	45.49% <42.85%> (-0.18%)	⬇️
crates/ditto-ast/src/expression.rs	83.33% <50.00%> (-2.67%)	⬇️
crates/ditto-ast/src/name.rs	77.77% <50.00%> (-2.23%)	⬇️
crates/ditto-fmt/src/expression.rs	98.86% <71.42%> (-0.76%)	⬇️
crates/ditto-cst/src/parser/expression.rs	99.59% <91.66%> (-0.41%)	⬇️
...ditto-checker/src/module/value_declarations/mod.rs	99.20% <100.00%> (+<0.01%)	⬆️
crates/ditto-checker/src/typechecker/mod.rs	96.26% <100.00%> (-0.15%)	⬇️
crates/ditto-checker/src/typechecker/pre_ast.rs	96.77% <100.00%> (+0.10%)	⬆️
... and 8 more

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