Implement an option to store repeated fields in Vec

[stepancheg]

Currently repeated fields of message type are generated as RepeatedField<Message>.

That helps with high performance code: allocated memory for one message can be reused for next message, but practically people rarely reuse messages.

Rust-protobuf needs an option to generate repeated fields as Vec<Message>, which could be turned on by default later.

[stepancheg]

Implemented in 87c156d.