This allows the main TR module not to explicitly
depend on the contract library. Each exported name
can be redirected to either the original name (for a
typed client) or to another macro (for an untyped
client) which expands to a `local-require` for the
relevant submodule (named `#%contract-defs`).
Thanks to Matthew for the initial idea and an
implementation in plai-typed that this is based on.
Recent changes to contract generation have made this
`local-require` unnecessary. Using `require` is preferable
since `local-require` can interact poorly with submodules.
