Polymorphism isn't specializing for some fairly simple polymorphic functions

[vdka]

Foo :: fn(a: ($T) -> bool) -> void {
}

main :: fn() -> void {
    Foo(fn(a: i64) -> bool { return 1 == 2 })
}

Failed to specialize parameter a (type ($T) -> bool) with 'fn(a: i64) -> bool { return 1 == 2 }' (type (i64) -> bool)

See the long out of date branch polymorphic-functions for some insight into fixing this