Add vec::swap_remove

[eholk]

It'd be nice to have a function that removes an element from the middle of a vector and replaces it with the element at the end. It's important that it not require a copy bound.

One place this would be useful is in pipes::select, which must remove the selected endpoint from the list. Currently we have a linear time operation to rebuild the vector, while swap_remove would make it a constant time operation. This could be important for performance in, for example, servo.

[bblum]

swap_shift would be useful too - preserve order, but be O(n).

er, on second thought, that should probably just be called remove!

[bblum]

that was about 20 minutes' work. that's what i like from my E-easy bugs..!

[eholk]

Awesome, thanks!