- Refactor error codes to use strings instead of ints.
- Simplify error messages throughout.
- Ensure location set on all expressions. There were a couple locations
in the parser/compiler where locations were not being set.
- Fallback to rule location in topdown in case location not set. This
ensures that users get useful locations for API requests with paths
that refer to virtual docs exactly.
Also add Find function to ast.Value. Useful for extracting values
dynamically. Eventually can support JSON pointers.
Fixes#237