DataRow without allocation; DataRow as Collection; RowDescription top level

This patch was authored and released by @fabianfett.

This is a cherry pick of #188.

Modifications

• DataRow and RowDescription have been moved out of the PSQLBackendMessage namespace. This allows us to mark them as @inlinable or @usableFromInline at a later point, without marking everything in PSQLBackendMessage as @inlinable
• DataRow does not use an internal array for its columns anymore. Instead all read operations are directly done on its ByteBuffer slice.
• DataRow implements the Collection protocol now.

Result

One allocation fewer per queried row.