[enhancement] APFloat parametrics cleanup

### What's hard to do? (limit 100 words)

apfloat could use some (breaking) changes to improve consistency and usability of parametrics.

For example, we have:

`pub fn to_uint`

which means the APFloat parametrics can be inferred with just supplying the `RESULT_SZ`.

But the signed version looks like:

`pub fn to_int`

which means you need to specify all 3. I think this is arguably worse than both using the latter form, because then at least you are forced to specify all 3 parametrics in the same order.

We also have:

`pub fn round`.

This one is a bit unfortunate, because we need round style to be last to take a default, but the other two parametrics can always be inferred. This restricts inference to the no parametrics specified case. To improve this one might take a larger language change.

Another example:

`pub fn downcast`

Unlike most apfloat parametric lists (including `APFloat` itself, this takes`FRACTION_SZ` first, then `EXP_SZ`.

### Current best alternative workaround (limit 100 words)

I usually discover these due to type inference errors and then carefully review the API.

### Your view of the "best case XLS enhancement" (limit 100 words)

Parametrics should be consistent and allow automatic inference as much as possible.