Prevent eval_const_expr_partial hides prior error in expr_cast arm.

This will help not to meet confusing errors.
In issue #5873, the error was "expected constant expr for vector length: Can't cast str to int".
It was originally "expected constant expr for vector length: Non-constant path in constant expr"
This patch make the original error to be printed.
Update AUTHORS.txt w/ Brett Cannon, Diggory Hardy, Jack Moffitt, James Miller
libcore,std,syntax,rustc: move tests into `mod tests`, make them private (no pub mod or pub fn).
librustdoc: move tests into dedicated tests module.
auto merge of #5894 : catamorphism/rust/authors, r=catamorphism
update RELEASES.txt
iterator: use an IteratorUtil trait
libcore: missed an import for a test
auto merge of #5899 : catamorphism/rust/copy-cops, r=catamorphism
auto merge of #5895 : huonw/rust/no-pub-tests, r=thestinger

This patch is a sledge hammer that moves all tests into `#[cfg(test)] mod test { .. }`, and makes them private, there were several instances of `pub mod tests { #[test] pub fn ... } `.

(The reason for this is I was playing with using `syntax` to index code ([result so far](http://www.ug.it.usyd.edu.au/~hwil7821/rust-api/)) and it was getting some junk from the tests.)

The rustdoc commit is particularly brutal, so it's fine if that one isn't landed.
auto merge of #5901 : thestinger/rust/iterator, r=sanxiyn

Can now use them like `x.transform(|i| i + 3).zip(y.filter(|i| i % 2)`.
libcore: vec::windowed iterates, not allocates.
auto merge of #5904 : huonw/rust/core-vec-windowed, r=thestinger

And add some documentation.
auto merge of #5890 : youknowone/rust/const-eval, r=catamorphism

This will help not to meet confusing errors.
In issue #5873, the error was "expected constant expr for vector length: Can't cast str to int".
It was originally "expected constant expr for vector length: Non-constant path in constant expr" (though still invalid error).
This patch make the original error to be printed.
rustc: One less copy
rustc: Anti-copy police

In this case, some copies are still necessary to convert from
a mutable to an immutable @-box. It's still an improvement,
I hope.