fix: parser would hand on function type with colon in it

[asterite]

Description

Problem

@nventuro Found that this snippet crashed VSCode:

impl<T, let MaxLen: u32> BoundedVec<T, MaxLen> {
    fn for_each<U, Env>(self, f: fn[Env](value: T, index: u32)) {
        for i in 0..MaxLen {
            if i < self.len() {
                f(self.get_unchecked(i), i);
            }
        }
    }
}

Summary

It turns out that a function type like this: fn(value: T) crashed the compiler. The reason is that parsing an fn parameter called parse_type_or_error and considered that an error if it returned UnresolvedTypeData::Error. The problem is that that method returned UnresolvedTypeData::Unspecified in that case. I think at one point I accidentally changed it to return Unspecified in order to reuse a function. The name is parse_type_or_error so returning Error is the correct thing to do, and it also solves the issue.

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Compilation Sample

Program	Compilation Time	%
sha256_regression	0m1.471s	1%
regression_4709	0m1.583s	3%
ram_blowup_regression	0m17.404s	4%

[github-actions]

Peak Memory Sample

Program	Peak Memory	%
keccak256	81.38M	0%
workspace	121.97M	-1%
regression_4709	333.59M	0%
ram_blowup_regression	2.55G	0%