Move away from using Connect `Struct` internally to represent the key.

Move to something more akin to `GenericRow`, i.e. just an array of column values.

Aside from moving us one step closer to removing Connect types from our core system, it also has a performance benefit. `Struct` validates the type of ever field being set, which is expensive, especially for complex types. The Serde have already ensured the types match, so why pay the cost?

No files or tests are named. Trace internal uses of Connect Struct for keys and compare them with the GenericRow-style array representation, starting at the key-handling and Serde paths. Done means the internal key representation no longer depends on Connect Struct while preserving existing key values and Serde behavior.

Written by the indexing model from the issue text.