We use "keypair" in identifiers (local vars and function names) but
`KeyPair` - one of them is wrong.
Elect to follow upstream and define keypair as a single word i.e., use
`Keypair` for type name and `keypair` in identifiers.
This patch can be reproduced mechanically by doing two
search-and-replace operations on all files excluding the CHANGELOG
- Replace "KeyPair" with "Keypair"
- Replace "key_pair" with "keypair"
33747bb16f