support for ref extern types

[ottoville]

Fixes #299

Added

• Support non-null WASM reference types ((ref extern)) on imported and exported
  function arguments and results, including JS string builtins.

Rust types Resource, &Resource and &mut Resource are preprocessed into (ref extern) (new type introduced by this PR)
Rust types Option<Resource>, Option<&Resource> and Option<&mut Resource> are preprocessed into (ref null extern) (aka. externref, current behavior)

Changed

• Breaking: Derive WASM nullability from Rust types. Resource, ResourceCopy and
  their references now use (ref extern); Option forms use (ref null extern).

Fixed

• Handle optional mutable resource references in generated import wrappers.
• Recognize debug import guards after argument spills in the stack prologue.