let bindings for ports and parameters

[rachitnigam]

We should add support for let-bindings to name ports and parameters.

Ports

A port let-binding is the same as a bundle of size one:

let p: [G, G+1] 32;
...
if cond {
  p = x.out
} else {
  p = y.out;
}
foo.in = p

Much like bundle types, a port definition of this form needs to have exactly one driver active within any given branch.
With the IR-flow, we can desugar port let-bindings into simple bundle definitions. For now, we'll require that the user provide the type for the port definition (like bundles) and eventually remove the restrictions once we have full type inference.

Parameters

A parameter let-binding allows us to name parameter expressions:

let #P = #K+1;
...
if #P > 10 { ... } else { ... }

This can make some of the code we write today nicer.