Make `reflect::Map::drain` and `reflect::List::drain` take a mutable borrow instead of `Box<Self>`.

## What problem does this solve or what need does it fill?

Draining a map or list does not require an owned value - it only requires a mutable borrow. However our reflection API does require consuming an owned value.

## What solution would you like?

Make `drain` for `Map` and `List` take a mutable borrow!