Revisit the function args/re-interpret expression as pattern

[FreeMasen]

Currently to handle function arguments there is a step that re-interprets an expression as a pattern. This feels pretty messy, the function arg struct could handle this more effectively

[FreeMasen]

Mostly covered in v0.6.0, though it would be good to give this another good once over.

[FreeMasen]

As a note, it may be possible to remove the FuncArg::Expr and require they all be Pats