extra: Put the nail in the coffin, delete libextra

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

src/test/compile-fail/deriving-span-Hash-tuple-struct.rs

@@ -11,7 +11,6 @@
 // This file was auto-generated using 'src/etc/generate-keyword-span-tests.py'
 
 #[feature(struct_variant)];
-extern crate extra;
 extern crate rand;