[breaking-change]

cc #11586

This pull request:

- Merges the `Round` trait into the `Float` trait, continuing issue #10387.
- Has floating point functions take their parameters by value.
- Cleans up the formatting and organisation in the definition and implementations of the `Float` trait.

More information on the breaking changes can be found in the commit messages.

Now with proper checking of enums and allows unsized fields as the last field in a struct or variant. This PR only checks passing of unsized types and distinguishing them from sized ones. To be safe we also need to control storage.

Closes issues #12969 and #13121, supersedes #13375 (all the discussion there is valid here too).

This currently requires linking against a library like libquadmath (or
libgcc), because compiler-rt barely has any support for this and most
hardware does not yet have 128-bit precision floating point. For this
reason, it's currently hidden behind a feature gate.

When compiler-rt is updated to trunk, some tests can be added for
constant evaluation since there will be support for the comparison
operators.

Closes #13381

This currently requires linking against a library like libquadmath (or
libgcc), because compiler-rt barely has any support for this and most
hardware does not yet have 128-bit precision floating point. For this
reason, it's currently hidden behind a feature gate.

When compiler-rt is updated to trunk, some tests can be added for
constant evaluation since there will be support for the comparison
operators.

Closes #13381

And allow the last field of a struct or variant to be unsized.

Replaced "len" by "length", to match the given code example.

Was killing performance of selector matching in Servo.

r? @alexcrichton (or anyone)

This pr updates the "Additional tutorials" link in the scope section to match the "What next?" link in the table of contents.

* fix official shootout test (spacing)
* use libgreen to improve performances
* simplify and modernize code
* remove warnings