Modify method dispatch to use trait machinery

[nikomatsakis]

Currently, method dispatch just compares the self type and -- if it matches -- assumes that the trait matches. It ought to use a strategy akin to what is used during trait matching. This probably involves some minor refactoring of the select trait to allow method dispatch to "hook in" to the same code.

Part of trait reform #5527.

Nominating.

[nikomatsakis]

Done