Allow irrefutable patterns as function parameters

[jmackie]

This makes working with type "wrappers" (a.k.a newtype*) much more ergonomic by removing the need for single arm match expressions.

Before:

module Wrapper exports (Wrapper, unwrap);

type Wrapper(a) = Wrapper(a);

unwrap = fn (wrapper) -> 
    match wrapper with
    | Wrapper(a) -> a
    end;

After:

module Wrapper exports (Wrapper, unwrap);

type Wrapper(a) = Wrapper(a);

unwrap = fn (Wrapper(a)) -> a;

This becomes especially powerful once we support a richer pattern syntax 👌.

^{*I know "wrappers" like this aren't really newtypes as they have a runtime cost...}

[codecov]

Codecov Report

Merging #94 (847691e) into main (ee473a5) will increase coverage by 0.00%.
The diff coverage is 84.61%.

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   73.89%   73.90%           
=======================================
  Files         111      111           
  Lines        8314     8319    +5     
=======================================
+ Hits         6144     6148    +4     
- Misses       2170     2171    +1     

Impacted Files	Coverage Δ
crates/ditto-checker/src/result/type_error.rs	95.83% <0.00%> (-3.10%)	⬇️
...ates/ditto-checker/src/typechecker/substitution.rs	77.47% <ø> (ø)
crates/ditto-cst/src/get_span.rs	91.02% <ø> (-0.44%)	⬇️
crates/ditto-fmt/src/has_comments.rs	46.37% <ø> (+0.27%)	⬆️
crates/ditto-checker/src/typechecker/mod.rs	82.07% <65.21%> (-1.31%)	⬇️
crates/ditto-codegen-js/src/convert.rs	96.44% <96.42%> (-0.03%)	⬇️
crates/ditto-ast/src/expression.rs	81.63% <100.00%> (+4.43%)	⬆️
...ditto-checker/src/module/value_declarations/mod.rs	91.25% <100.00%> (+0.16%)	⬆️
crates/ditto-checker/src/typechecker/pre_ast.rs	100.00% <100.00%> (ø)
crates/ditto-fmt/src/expression.rs	100.00% <100.00%> (+0.67%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more