where clauses in function definitions leak out and impact global definitions

[eminence]

Consider this function:

fn foo(x: Scalar) -> Scalar = x + something where something = 1

Ideally we'd like and expect that something is locally scoped to this function definition. But it's name leaks out and prevents the name something from being used in some contexts (like unit definitions):

>>> unit something
error: identifier clash in definition
  ┌─ <input:2>:1:51
  │
1 │ fn foo(x: Scalar) -> Scalar = x + something where something = 1
  │                                                   --------- Previously defined here
  │
  ┌─ <input:4>:1:6
  │
1 │ unit something
  │      ^^^^^^^^^ identifier is already in use