Add unique codes for each error message

[richfitz]

This PR adds a unique error code for each error that we throw. For example E1007 means that the user has provided a default argument to parameter() that is too complicated for us to work this. Each call to odin_parse_error includes a code, and each code has an entry in the errors vignette. There's also a basic odin_error_explain function which will (currently) link to the vignette.

This is all likely to expand over time, some future directions are described in comments.

Merge after #4 as it contains those commits

[weshinsley]

Looks great! (Especially the RStudio integration... spectacular...)