The `is_power_of_two()` method of the `UnsignedInt` trait currently returns `true` for `self == 0`. Zero is not a power of two, assuming an integral exponent `k >= 0`. I've therefore moved this functionality to the new method `is_power_of_two_or_zero()` and reformed `is_power_of_two()` to return false for `self == 0`.

To illustrate the usefulness of the existence of both functions, consider `HashMap`. Its capacity must be zero or a power of two; conversely, it also requires a (non-zero) power of two for key and val alignment.

Also, added a small amount of documentation regarding #18604.

Vec panics when attempting to reserve capacity > int::MAX (uint::MAX / 2).

parse_ty() no longer takes a boolean parameter. quote_ty! implementation has not yet been modified accordingly. 

As a matter of fact, quote_ty! was not covered by tests. One test (called qquotes) references it, but it has been ignored for nearly one year and now need heavy refactoring.

quote_token.rs seemed like a good place to test quote_ty!, many other quote_*! macros were asserted there.

r? @alexcrichton 

Rewrite how the HRTB algorithm matches impls against obligations. Instead of impls providing higher-ranked trait-references, impls now once again only have early-bound regions. The skolemization checks are thus moved out into trait matching itself. This allows to implement "perfect forwarding" impls like those described in #19730. This PR builds on a previous PR that was already reviewed by @pnkfelix.

r? @pnkfelix 

Fixes #19730 

not dug deeply into what is going on here, although the errors ARE
somewhat surprising.

though it includes a `fn()`. This is really a more general
problem but I wanted to ensures that `bytes` in particular
remains working due to #12677.

lifetimes. This currently causes an ICE; it should (ideally) work, but
failing that at least give a structured error. For the purposes of
this PR, though, workaround is fine.

read (`//!` is intrusive) and annoying to edit (must maintain a prefix
on every line). Since the only purpose of a `doc.rs` file is to have a
bunch of text, using `/*!` and `*/` without indentations seems
appropriate.