std: stop `vec!()` warning about unused mutability.

If no arguments are given to `vec!` then no pushes are emitted and
so the compiler (rightly) complains that the mutability of `temp` is
never used.

This behaviour is rather annoying for users.
change FVN hash function to the FVN-1a variant
auto merge of #12749 : tedhorst/rust/master, r=huonw

Minor change to the FVN hashing function based on recommendation on: http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash

cc @alexcrichton
auto merge of #12750 : liigo/rust/De-extern-mod, r=alexcrichton

rename ast::ViewItemExternMod to ast::ViewItemExternCrate
rename rustdoc::clean::ExternMod to rustdoc::clean::ExternCrate
because `extern mod` is out.
auto merge of #12753 : huonw/rust/vec-macro, r=cmr

If no arguments are given to `vec!` then no pushes are emitted and
so the compiler (rightly) complains that the mutability of `temp` is
never used.

This behaviour is rather annoying for users.