Rename the Uint256KeyWrapper struct

Summary: This is not a wrapper to a key, this is actually a type to be used as a 256 bits radix key. There is no change in behavior.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11473