feat(rpc): extend search path, remove cd

[pnwamk]

Disable the ability for clients to change the current working directory of a running server - instead make it trivial to extend the search path dynamically. See #1081

• add a test
• verify the eval server is okay with this feature (it did not have the ability to change a working directory before... but extending the search path seems harmless and unlikely to violate eval server restrictions)
• investigate if saw-remote-api also needs this feature

[pnwamk]

@atomb In this PR I've added the ability for clients to extend the CRYPTOLPATH the server uses to find modules for both cryptol-remote-api and cryptol-eval-server. I just want to double check this functionality doesn't conflict with the goals of the latter.

[atomb]

This looks good to me.