Pre-RFC: Downgradable mutable borrows

[cg909]

@steffahn

I have a minor concern that &'long mut<'short> T: Send implementation, which – if this transparently expands the existing &mut T type – just requires T: Send, no T: Sync might possibly be problematic, but I couldn’t come up with any scenario yet how this can turn into an actual soundness issue.

I think it should still be safe. To use a reference as a &'long mut<'short> argument 'short still must be valid, so this doesn't change in respect to the existing &mut T. Only if immutable and mutable references were unified e.g. with something like &'long mut<'!> T it would become problematic.