There's a lot of stuff wrong with the representation of these types:
TyFnDef doesn't actually uniquely identify a function, TyFnPtr is used to
represent method calls, TyFnDef in the sub-expression of a cast isn't
correctly reified, and probably some other stuff I haven't discovered yet.
Splitting them seems like the right first step, though.

rather than being implicit quasi-subtyping. Nothing good can come out
of quasi-subtyping.

Fixes #28839

Currently, we're generating adjustments, for example, to get from &[u8]
to &[u8], which is unneeded and kicks us out of trans_into() into
trans() which means an additional stack slot and copy in the unoptimized
code.

Closes #26205.

This commit finalizes the work of the past commits by fully moving the fulfillment context into
the InferCtxt, cleaning up related context interfaces, removing the Typer and ClosureTyper
traits and cleaning up related intefaces

Unlike coercing from reference to unsafe pointer, coercing between two
unsafe pointers doesn't need an AutoDerefRef, because there is no region
that regionck would need to know about.

In unoptimized libcore, this reduces the number of "auto_deref" allocas
from 174 to 4.

Use camel-case naming, and use names which actually make sense in modern Rust.

Added LvaluePreference::from_mutbl

Closes #15609

As per RFC#520 the syntax for arrays has changed,
this changes the remaining comments to reflect
the new syntax.

+ lots of rebasing

so that it is closer to standalone.

context private.