This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.
Modifies parsing to return strongly-typed `Thunk`s, which contain both
the function implementation, as well as the correct number of arguments.
This moves unknown function and function argument count mismatch errors
to parse time.
