This is caused by StrVector having a generic implementation for &[S]
and so #5898 means that method resolution of ~[~[1]].concat() sees that
both StrVector and VectorVector have methods that (superficially) match.

They are now connect_vec and concat_vec, which means that they can actually be
called.

copies can just be done explicitly: `xs.transform(|x|x.clone())`

replaced with mutable implementations of Iterator

replaced with `xs.mut_iter().enumerate()`

This fixes the strange random crashes in compile-fail tests.

This reverts commit 96cd61ad.

Conflicts:
	src/librustc/driver/driver.rs
	src/libstd/str.rs
	src/libsyntax/ext/quote.rs

You can still initialize multiple variables at once with "let (x, y) = (1, 2)".

Closes #6607

Adds documentation for various things that I understand.
Adds #[allow(missing_doc)] for lots of things that I don't understand.

This only changes the directory names; it does not change the "real"
metadata names.