auto merge of #16603 : SiegeLord/rust/nullable, r=alexcrichton

A few reasons:

* `Nullable` is basically unused, save for one argument in the `glob` function in `liblibc`, so this change likely impacts nobody negatively. The constructors are never used, and I don't foresee people using them. The people implementing the glob functionality don't seem to be reaching for this POSIX `glob` function, so it seems unlikely to be used heavily.
* At the same time, the old name, `Some`, needlessly conflicted with the same re-exported name in the prelude, which impacted everybody who use glob imports with `libc`. Changing it to something else would simplify things greatly for those people.
* `NotNull` seemed like the best option (others included `Just`, `Valid`, etc. which all had somewhat different connotations than what this type was going for (even `Some` doesn't quite seem like the opposite of `Null`)). Other options included removing the type completely and adding a special, `glob`-specific type. This latter approach doesn't seem future-proof.

Overall, I feel like this is a mildly positive change.

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