Call typechecker from isolated frame

[ojw28]

The version of TypeGuard that JaxTyping depends on grabs the calling frame's locals using sys._getframe(1).f_locals, and returns a wrapping function that holds a reference to these locals. JaxTyping currently calls the typechecker from a frame that contains some locals. Of these, full_fn and param_fn are problematic because they, in combination with what TypeGuard is doing, end up creating reference cycles.

This commit calls the typechecker from an isolated frame in JaxTyping. This means f_locals of the calling frame doesn't contain anything that can end up creating cycles.

It's somewhat debatable whether this should be considered a TypeGuard problem, but I think it would be tricky to fix there. TypeGuard appears to need a mutable version of the locals to see forward defined variables, so it's not viable just to copy them and remove anything that's problematic. Even if it were, TypeGuard doesn't really know which locals are problematic and not required. So pragmatically, changing JaxTyping to guard against the issue feels like the correct thing to do.

[patrick-kidger]

Yup, this makes sense to me / LGTM! Merged.