extra: Put the nail in the coffin, delete libextra

[alexcrichton]

This commit shreds all remnants of libextra from the compiler and standard
distribution. Two modules, c_vec/tempfile, were moved into libstd after some
cleanup, and the other modules were moved to separate crates as seen fit.

Closes#8784
Closes#12413
Closes#12576

[flaper87]

👏

[sfackler]

The rust-lang.org homepage will need to be updated for this.

[brson]

Perhaps we can find different homes for these modules later? I hate to think they got promoted to std just because they were the last modules standing.

[alexcrichton]

I think that c_vec should end up wherever c_str goes (they're pretty much the same thing), and tempfile is arguably a core-ish functionality, but it's sad to lose the true random directory names.

[huonw]

sad to lose the true random directory names

We could have the core rand functionality as super-secret inside libstd, and reexport it in librand.

[derekchiang]

Just wanted to point out that there are dead links to extra on the homepage and also the "other docs" page.

[alexcrichton]

See rust-lang/prev.rust-lang.org#22 for removal of the links.