Initial implementation of dyn*

[eholk]

This PR adds extremely basic and incomplete support for dyn*. The goal is to get something in tree behind a flag to make collaboration easier, and also to make sure the implementation so far is not unreasonable. This PR does quite a few things:

• Introduce dyn_star feature flag
• Adds parsing for dyn* Trait types
• Defines dyn* Trait as a sized type
• Adds support for explicit casts, like 42usize as dyn* Debug
  • Including const evaluation of such casts
• Adds codegen for drop glue so things are cleaned up properly when a dyn* Trait object goes out of scope
• Adds codegen for method calls, at least for methods that take &self

Quite a bit is still missing, but this gives us a starting point. Note that this is never intended to become stable surface syntax for Rust, but rather dyn* is planned to be used as an implementation detail for async functions in dyn traits.

Joint work with @nikomatsakis and @compiler-errors.

r? @bjorn3