Expose a safe low-level cursor API.

[nolanderc]

The LMDB cursor API is quite powerful; something similar should be exposed by heed. The RwIter API gets close, but is unsafe and does not handle seeking to arbitrary keys. The API should allow the following operations:

• Seeking to an arbitrary key.
• Seeking to the next/previous key-value pair.
• Reading the current key/value (could be part of the seeking operations).
• Inserting an arbitrary key-value pair (LMDB optimizes for cases where the key would end up close to where the cursor is currently pointing).

A full cursor API would also let us efficiently implement an API similar to HashMap::entry (see this comment).