Removing a lot of usage of '&const'
Remove mutability from unique boxes in the AST
auto merge of #7342 : alexcrichton/rust/assort-cleanup, r=cmr

This removes usage of `&const` throughout the standard libraries/compiler, and it removes some extraneous fields in the AST now that unique boxes always inherit their mutability.
minor vec cleanup

* hide the rustrt module in the docs
* remove the useless `traits` module wrapping the `Add` impl