Print reasons for subtyping mismatches

[kaleidawave]

When doing subtyping (initial declaration, assigning, passing argument as parameter, return type etc) the returned result is a SubTypeResult. The subtyping logic (in types/subtyping.rs) currently collects some of the reasons for mismatches when doing the checking and stores this as a NonEqualityReason.

However this reason is thrown away in many places and does not make it to diagnostics. This can be shown by running rg "SubTypeResult::IsNotSubType\(_".

It would be better to pass this via a field to many of the diagnostics. For example here

ezno/checker/src/context/mod.rs

Lines 1115 to 1120 in 5191329

	DoesNotMeetConstraint {
	variable_type: TypeStringRepresentation,
	variable_site: SpanWithSource,
	value_type: TypeStringRepresentation,
	value_site: SpanWithSource,
	},

There should be something like mismatch: MismatchExplanation. This can be turned into a user readable explanation of why the type error happened. This can be surfaced via adding labels in the Diagnostic::PositionWithAdditionalLabels.

The TypeStringRepresentation enum was designed to cover this, but I don't think it quite fits into this structure.