Reserve space in Vec::extend() before pushing elements

[andylokandy]

Reserve space by size_hint() in Vec::extend() before pushing elements.

[fitzgen]

Thanks for sending a pull request.

bumpalo::collections::Vec is a fork of std::vec::Vec and I don't really want to get into the business of changing its algorithm and implementation. so unless std::vec::Vec started doing this and we are matching its new behavior, I'd prefer not to diverge.

[TethysSvensson]

As far as I know, standard Vec is using all sorts of specialization magic to optimize it's extend and we are not.

[fitzgen]

Yeah that's fair.