wit: Disallow duplicate names between types and functions

[peterhuene]

Currently in wit, this is legal:

record foo {
}

foo: function(f: foo)

In the current component mode proposal, exports are used to name things, so a component with this interface would export a record interface type named foo and a function type named foo. As duplicate exports are illegal in WebAssembly components, this would fail validation.

Another reason to not allow this is that some language bindings might use a similar naming convention between type identifiers and function identifiers and such a binding generator would need to use an alternative (potentially unwieldy) name to prevent a collision

[esoterra]

Should names be unique across functions, resources, and types?
I'm working on a draft implementation and all three are handled separately at the moment.

[alexcrichton]

I think to accomodate the broadest variety of languages we'll be generating bindings into it's probably best to have everything in the same namespace and forbid any clashes.

[alexcrichton]

done in #867