Finalize defaulted type parameters

This RFC proposes finalizing the design of defaulted type parameters with two changes:

- Using `_` to explicitly use a default
- Integrating defaults and inference

[Rendered view.](https://github.com/nikomatsakis/rfcs/blob/defaulted-type-params/active/0000-defaulted-type-params.md)
s/is not other constraint/is not otherwise constrained/