Puts implementations in bins hashed by the fast-reject key, and
only looks up the relevant impls, reducing O(n^2)-ishness

Before: 688.92user 5.08system 8:56.70elapsed 129%CPU (0avgtext+0avgdata 1208164maxresident)k, LLVM 379.142s
After: 637.78user 5.11system 8:17.48elapsed 129%CPU (0avgtext+0avgdata 1201448maxresident)k LLVM 375.552s

Performance increase is +7%-ish

Introduces new variants and types in syntax::ast, middle::ty, and middle::def.

comments and also leave the entries in the variance tables for now.

combine normalization with this check so that we also skip the
default method if normalization fails. Fixes #23485.

us to construct trait-references and do other things without forcing a
full evaluation of the supertraits. One downside of this scheme is that
we must invoke `ensure_super_predicates` before using any construct that
might require knowing about the super-predicates.

predicates and renaming some things.

all parameter environments are normalized. Correspondingly, stop
normalizing predicates we extract out of the environment. Fixes #21664.

I cleaned up comments prior to the 1.0 alpha release.

fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].

fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.

This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.

Part of #20013

[breaking-change]

check it more easily; also extend object safety to cover sized types
as well as static methods.  This makes it sufficient so that we can
always ensure that `Foo : Foo` holds for any trait `Foo`.