Have prefix based iterators

[capocasa]

LMDB natively has some operations to iterate over keys with a certain prefix. LimDB currently does not expose these.

The new Nim module std/critbittree as exactly such an API. Since LimDB tries to be close to the standard library for familiarity (and because the author of LimDB kind of digs stdlib style) this would be a great example to work by.

So we need to

1. have a look at which std/critbittree features related to prefixes overlap with LMDB functionality
2. Implement them

If you are reading this and might be interested in contributing (1) (and possibly later 2) you would be most warmly welcomed.