dbcrossbarlib: Translate table schemas and copy tables between databases

This is currently an internal library used by dbcrossbar, and it has an unstable API. We might someday decide to stabilize this as a public API that performs the same operations as the dbcrossbar CLI. It is not yet intended for use outside of that tool, and it may not always respect Semantic Versioning rules.

lib.rs:

A library for reading and writing table schemas in various formats.

At the moment, the most interesting type here is the schema module, which defines a portable SQL schema.