per pnkfelix's suggestion

suggested by arielb1

as suggested by arielb1

This includes the additional case suggested by arielb1.

It's just not useful. It also makes it hard to have tests that probe
internal state, since the interning number is very sensitive.

Dumping the number in the case of gensym is not ideal but will do for
now.

It's inefficient, and the substitution there doesn't account for the
extra regions used by NLL inference, so it's a bad thing to encourage.

As it happens all callers already know if they have a closure or not,
from what I can tell.

The overall format is now easier to read. Also, There is now graphviz
output, as well as a `#[rustc_regions]` annotation that dumps internal
state.

We also rename some of the files to conform to the
feature-gate-<feat_name>.rs pattern that is most common.

Renames only in this commit, and obviously
.stderr file additions.

ui tests are the future, especially since the
recent improvement where we have gained
checking and requiring of //~ERROR comments.

The tidy feature-gate test check is intended
to be 50% an actual insurance that there is
a check, and 50% to be a teacher that such
checks are required.

With this commit applied, newbies might
interpret stuff wrongly and create tests
that don't fail but succeed instead.

This is not what feature gate tests are
for though. Therefore, in a later step,
when only ui tests are allowed to be feature
gate tests, we will add checking to ensure
that a file marked as gate test is actually
required to be a compilation failure.

Right now implementing such a check is a
bit annoying as one needs to only do it
when the compile-fail test is in the
ui test suite :/.

the use of lazily evaluated alternatives when appropriate. These
comments are intended to echo a clippy lint on the same topic (see
https://rust-lang-nursery.github.io/rust-clippy/master/index.html#or_fun_call)