This adds parser support for the new lambda syntax. It does not remove
the existing syntax, nor will the new syntax be retained in formatting.
That will be done in a separate PR to keep the two respective PRs
relatively small and easy to review.
Remove branches on determining how let-bindings are introduced to the
scope. This is maybe a little more inefficient, but I think it is a huge
simplification.
One additional change this required was changing how fx suffixes are
checked. The current implementation would add additional constraints for
patterns in let bindings conditionally. However, this is unnecessary. I
believe it is sufficient to check the fx suffix by running the checks on
all introduced symbols after the type is well known (i.e. the body is
checked).
Type variables can only be used on functions (and in number literals as
a carve-out for now). In all other cases, a type variable takes on a
single, concrete type based on later usages. This check emits errors
when this is violated.
The implementation is to check the rank of a variable after it could be
generalized. If the variable is not generalized but annotated as a type
variable, emit an error.
Remove branches on determining how let-bindings are introduced to the
scope. This is maybe a little more inefficient, but I think it is a huge
simplification.
One additional change this required was changing how fx suffixes are
checked. The current implementation would add additional constraints for
patterns in let bindings conditionally. However, this is unnecessary. I
believe it is sufficient to check the fx suffix by running the checks on
all introduced symbols after the type is well known (i.e. the body is
checked).
