Mutable vector should be growable.

[spdegabrielle]

No description provided.

[spdegabrielle]

requested by @sorawee

[sorawee]

There are two parts for this:

1. I can't think of a reason why there would be an overhead in growable vector, but if there's one, we might want to keep fixed-size vector for those who want performance. However, growable vector should be provided as the default, with a sensible name that is not gvector.
2. But if not, we could also just provide growable vector alone.

[rmculpepper]

I disagree. There are lots of ways that code can break if a mutable vector unexpected changes size (presumably growable vectors are also shrinkable vectors!). Such bugs and potential bugs were a major part of the reason that Racket made pairs immutable.

IMO, resizable vectors should be a conveniently-available but distinct data type.

[sorawee]

OK, I'm sold on this. But we should pick a better name than gvector.