Rather than declaring some region variables to be constant, and
reporting errors when they would have to change, we instead populate
each free region X with a minimal set of points (the CFG plus end(X)),
and then we let inference do its thing. This may add other `end(Y)`
points into X; we can then check after the fact that indeed `X: Y`
holds.

This requires a bit of "blame" detection to find where the bad
constraint came from: we are currently using a pretty dumb
algorithm. Good place for later expansion.

After this change, impl Trait existentials are
desugared to a new `abstract type` definition
paired with a set of lifetimes to apply.

In-scope generics are included as parents of the
`abstract type` generics. Parent regions are
replaced with static, and parent regions
referenced in the `impl Trait` type are duplicated
at the end of the `abstract type`'s generics.

On "the parameter type `T` may not live long enough" error, point to the
parameter type suggesting lifetime bindings:

```
error[E0310]: the parameter type `T` may not live long enough
  --> $DIR/lifetime-doesnt-live-long-enough.rs:28:5
   |
27 | struct Foo<T> {
   |            - help: consider adding an explicit lifetime bound `T: 'static`...
28 |     foo: &'static T
   |     ^^^^^^^^^^^^^^^
   |
note: ...so that the reference type `&'static T` does not outlive the data it points at
  --> $DIR/lifetime-doesnt-live-long-enough.rs:28:5
   |
28 |     foo: &'static T
   |     ^^^^^^^^^^^^^^^
```

Instead of requesting the region maps for the entire crate, request for
a given item etc. Several bits of code were modified to take
`&RegionMaps` as input (e.g., the `resolve_regions_and_report_errors()`
function). I am not totally happy with this setup -- I *think* I'd
rather have the region maps be part of typeck tables -- but at least the
`RegionMaps` works in a "parallel" way to `FreeRegionMap`, so it's not
too bad. Given that I expect a lot of this code to go away with NLL, I
didn't want to invest *too* much energy tweaking it.

Make a `CodeExtent<'tcx>` be something allocated in an arena
instead of an index into the `RegionMaps`.

Instead, thread around `Option<CodeExtent>` where applicable.

The warnings have already reached stable

The test rfc1592_deprecated is covered by `bad_sized` and
`unsized6`.

Fixes #33242
Fixes #33243

this commit should be reverted after a release cycle

this makes the code cleaner

This increases regionck performance greatly - type-checking on
librustc decreased from 9.1s to 8.1s. Because of Amdahl's law,
total performance is improved only by about 1.5% (LLVM wizards,
this is your opportunity to shine!).

before:
576.91user 4.26system 7:42.36elapsed 125%CPU (0avgtext+0avgdata 1142192maxresident)k
after:
566.50user 4.84system 7:36.84elapsed 125%CPU (0avgtext+0avgdata 1124304maxresident)k

I am somewhat worried really need to find out why we have this Red Queen's
Race going on here. Originally I suspected it may be a problem from RFC1214's
warnings, but it seems to be an effect from other changes.

However, the increase seems to be mostly in LLVM's time, so I guess
it's the LLVM wizards' problem.

`TransitiveRelation`