This field allows SMIR consumers to identify which locals correspond to
argument locals. It simply exposes the arg_count field from the MIR
representation.

This commit hides the arg_count field in Body and instead exposes more
stable and user-friendly methods to get the return and argument locals.
As a result, Body instances must now be constructed using the `new`
function.

This is particularly helpful for the ui tests, but also could be helpful
for Stable MIR users who just want all the locals without needing to
concatenate responses

The word internal has connotations about information that's not exposed.
It's more accurate to say that the remaining locals apply only to the
inner part of the function, so I'm renaming them to inner locals.

The latest locals() method in stable MIR returns slices instead of vecs.
This commit also includes fixes to the existing tests that previously
referenced the private locals field.