Remove erroneous reference to the base `Symbol` when mapping Rust types (#1439)

`.addReference(this)` adds a reference to the `Symbol` on which
`.mapRustType` was called. This is correct only when `f` is a function
that _wraps_ its input `RustType`; for example, when `f` wraps it in a
`Box` or constructs a `Vec`. However, the code is incorrect for an
arbitrary `f`; for example, when `f` _swaps_ the type.