Split the type context into a global and a local (inference-only) one.
After this change, each `InferCtxt` creates its own local type interner for types with inference by-products.
Most of the code which handles both a global and a local interner uses `'gcx` and `'tcx` for them.
A reference to the type context in that situation (e.g. `infcx.tcx`) is `TyCtxt<'a, 'gcx, 'tcx>`.
The global type context which used to be `&'a TyCtxt<'tcx>` is now `TyCtxt<'a, 'tcx, 'tcx>`.
In order to minimize the number of extra lifetime parameters, many functions became methods.
Where possible (some inherent impls), lifetime parameters were added on the impl, not each method.
As inference by-products no longer escape their inference contexts, memory usage is lower.
Example of `-Z time-passes` excerpt for `librustc`, stage1 (~100MB gains):
Before "rustc: Split local type contexts interners from the global one.":
```
time: 0.395; rss: 335MB item-types checking
time: 15.392; rss: 472MB item-bodies checking
time: 0.000; rss: 472MB drop-impl checking
time: 1.140; rss: 478MB const checking
time: 0.139; rss: 478MB privacy checking
time: 0.024; rss: 478MB stability index
time: 0.072; rss: 478MB intrinsic checking
time: 0.038; rss: 478MB effect checking
time: 0.255; rss: 478MB match checking
time: 0.128; rss: 484MB liveness checking
time: 1.372; rss: 484MB rvalue checking
time: 1.404; rss: 597MB MIR dump
time: 0.809; rss: 599MB MIR passes
```
After:
```
time: 0.467; rss: 337MB item-types checking
time: 17.443; rss: 395MB item-bodies checking
time: 0.000; rss: 395MB drop-impl checking
time: 1.423; rss: 401MB const checking
time: 0.141; rss: 401MB privacy checking
time: 0.024; rss: 401MB stability index
time: 0.116; rss: 401MB intrinsic checking
time: 0.038; rss: 401MB effect checking
time: 0.382; rss: 401MB match checking
time: 0.132; rss: 407MB liveness checking
time: 1.678; rss: 407MB rvalue checking
time: 1.614; rss: 503MB MIR dump
time: 0.957; rss: 512MB MIR passes
```
**NOTE**: Functions changed to methods weren't re-indented to keep this PR easier to review.
Once approved, the changes will be mechanically performed.
However, indentation changes of function arguments are there - and I believe there's a way to hide whitespace-only changes in diffs on GitHub.
c7ab8840c8