Fixed option_env! type

The type of the result of option_env! was not fully specified in the
None case, leading to type check failures in the case where the variable
was not defined (e.g. option_env!("FOO").is_none()).
rustc: Give a hint when a static method call has fewer than expected type parameters

In this case, it's likely to be that the user forgot the `self` type, so
say so.

Closes #4096
auto merge of #8477 : catamorphism/rust/issue-4096, r=msullivan

r? @msullivan ...e parameters

In this case, it's likely to be that the user forgot the `self` type, so
say so.

Closes #4096
auto merge of #8439 : huonw/rust/hashset-clone, r=cmr

Closes #5581.
auto merge of #8440 : sfackler/rust/env-fix, r=pcwalton

The type of the result of option_env! was not fully specified in the
None case, leading to type check failures in the case where the variable
was not defined (e.g. option_env!("FOO").is_none()).

Also cleaned up some compilation warnings.
rustc: allow @ as-patterns to move when the sub-pattern contains no bindings.

A pattern like `foo @ Foo(Bar(*), _)` should be legal, even if `foo` moves,
since the subpatterns are purely structural.

Fixes #3761.
std: Move the iterator param on FromIterator and Extendable to the method.

If they are on the trait then it is extremely annoying to use them as
generic parameters to a function, e.g. with the iterator param on the trait
itself, if one was to pass an Extendable<int> to a function that filled it
either from a Range or a Map<VecIterator>, one needs to write something
like:

    fn foo<E: Extendable<int, Range<int>> +
              Extendable<int, Map<&'self int, int, VecIterator<int>>>
          (e: &mut E, ...) { ... }

since using a generic, i.e. `foo<E: Extendable<int, I>, I: Iterator<int>>`
means that `foo` takes 2 type parameters, and the caller has to specify them
(which doesn't work anyway, as they'll mismatch with the iterators used in
`foo` itself).

This patch changes it to:

    fn foo<E: Extendable<int>>(e: &mut E, ...) { ... }