Some types used for encryption related tasks need to be used across more
internal crates than expected, so a few have been moved and renamed for
clarity.
* Rename the key_algorithms module in ruma-identifiers-validation to crypto_algorithms
* Move ruma_events::Algorithm to ruma-identifiers-validation and rename it EventEncryptionAlgorithm
* Move DeviceKeys from ruma-client-api to ruma-common
At the same time, this commit makes `ServerName` a newtype around str so
other identifier types can borrow out their server name part as a
`&ServerName`. This technique works for `ServerName` because it keeps no
additional metadata. For the other identifier types to support being
created in borrowed form from a string slice, custom DSTs first have to
be added to Rust.
