RepeatedField should implement std::iter::Extend

[qwandor]

As of 2.24.1, there doesn't seem to be an easy way to append multiple elements to a RepeatedField which already contains some elements. The simplest way to support this would be for it to implement std::iter::Extend, like Vec does.

[stepancheg]

Pushed to 2.25 branch (1611f9d).

Did not push to master since there's no RepeatedField in the next version of rust-protobuf.

[stepancheg]

Published 2.25.0.