Should vec::reserve reserve the exact amount or in powers of two?

[brson]

Currently it's exact, but there are places in vec that use it in conjuction with next_power_of_two to do it the other way, and #2062 also wants to reserve in powers of two.

Clearly there should be two different versions. Should the default be exact? If so, what should the next_power_of_two version be called?

[brson]

Jesse suggested reserve_at_least which I think is good. Think I will go ahead and add that and update the documentation on vec::reserve to indicate the exact behavior. Do the same for str::reserve probably.

[brson]

Implemented.