Implement revised coercion rules

[aturon]

Tracking issue for RFC 401.

[aturon]

Nominating: this contains backwards-incompatible changes.

For triage purposes, we probably want to pull out the DST coercions part of this as P-High.

[nrc]

Summary of changes to make from the RFC:

• Add cast from unsized slices to raw pointers (&[V] to *V);
• allow coercions as casts and add lint for trivial casts - Allow casts wherever implict coercion is allowed #18601;
• ensure we support all coercion sites;
• * remove [T, .. n] to &[T]/*[T] coercions - Disallow coercions from [T, ..n] to &[T] #18645;
• add raw pointer coercions - Add coercions from *mut to *const and from &mut to *const. #19766;
• add sub-trait coercions - rust-lang/rfcs#5665, Tracking issue for dyn upcasting coercion #65991;
• add unsized tuple coercions - Tracking issue for unsized tuple coercion #42877;
• add all transitive coercions - Allow transitive coercions #18602;
• receiver coercions - add referencing to raw pointers;
• * receiver coercions - remove double referencing for slices - Remove the double auto-ref on arrays/strings as receivers #19761;
• * remove function coercions, add function type polymorphism - Change function/closure coercions to polymorphism #18599;
• add DST/custom coercions - DST/custom coercions #18598.

We should double check that these changes actually get us to the right destination as defined by the RFC.

I filed bugs for the larger pieces of work, the rest are pretty minor and can be tracked here. Those marked with an asterisk are backwards incompatible.

[nrc]

Assigning myself for misc back-compat parts of this, I don't intend to do the rest right now.

[pnkfelix]

Assigning P-backcompat-lang, 1.0.