chore: improve error message for seq type mismatch

[alex-chew]

Fixes #1887.

This adds a wrapper to "lazily" retrieve the string representation of a sequence's element type that is unresolved while the type constraint (and its error message) is being constructed.

For simplicity, instead of the originally proposed message

s[0] has type T instead of the expected char (seq<T> is not compatible with the expected seq<char> aka string)

this PR changes the sequence-type-mismatch error message to

selected element has type T which is incompatible with expected type char (seq<T> is incompatible with seq<char>)

when a type mismatch arises from a single selection (e.g. s[0]).

When a type mismatch arises from a "multi-selection" (e.g. s[0..2]), the expected type is an InferredTypeProxy that may not resolve to a sequence type at all, in which case we can't speak of the expected sequence type's type argument. For that reason the multi-selection type mismatch error message makes no mention of the expected type argument:

multi-selection has type seq<T> which is incompatible with expected type seq<char>

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[cpitclaudel]

Looks good, but: are you actually using the new overload?

[cpitclaudel]

One more small thing that I just caught; sorry! IIUC the problem isn't ToString, it's .AsSeqType.Arg, right?

[cpitclaudel]

🎉