Rework method chaining system of Rust generator

[QuantumSegfault]

Follow up to #1586

After testing, #1586 had some major flaws in practice

• In contrary to Rust idioms, the method chaining was done using &Self, which makes things like passing a method chain into a parameter expecting an own more cumbersome than necessary.
• It was a blunt instrument, being at best minor pollution, and at worst, a hindrance. Especially once the first point is addressed, forcing all void returns to be owning self (and by extension take owning self) was not practical.

This replaces the --enable-method-chaining switch with a --chainable-methods parameter, that works similar to the current async filter. You can choose to opt-in to it for all methods, for all methods within a particular resource, or just for a particular method (or any combination of those).

Additional, methods with method chaining enabled now take owning self instead of the default borrowing &self, and are -> Self rather than -> &Self. The ABI remains unaffected.

[QuantumSegfault]

@alexcrichton

Requesting review.