[Parameterized modules] Add a `hw.int<T>` type

@lattner is already working on this.

Since Oct 5, 2021.

The next step in the journey for parameterized modules is to add parameterized types. The first one that makes sense to add is a parameterized integer type. Adding this is straight-forward now that parameter expressions are attributes, it roughly looks like:

1) Add the type. I suggest we make the width always be pinned to 32-bits to make the syntax cleaner and to line up with (non-System)Verilog better.
2) Assert that the parameter expressions are only-valid expressions at construction time.
3) We need operation verification to check that operations in a module only refer to valid parameterized types. The delta with step 2 is that we need to make sure that referenced parameters are correctly defined and typed within the module, e.g. reject:

```
hw.module @foo() {
someop: !hw.int // invalid type in foo module because p2 isn't defined here.
}
```

4) We need to handle parameter name legalization. This is not at all straight-forward given our current design for name legalization, because renaming a parameter would require changing the types of arbitrary operations, something that is a bit crazy. The right way to fix it is to complete Issue #1708, making the renaming completely a problem within the ExportVerilog pass.

5) [x] The problem with this is that we need to change HWExportModuleHierarchy around. I'll discuss this with richard.