Struct map is a mechanism to let Go code expose simple structs to Elvish code.
The difference between struct maps and maps is convenience for Go code; they
also have different performance characteristics, but since struct maps are
always quite small, the difference is not meaningful for Elvish's use cases.
As a result, there is no good reason that Elvish code needs to be aware of the
difference between struct maps and normal maps. Making them indistinguishable to
Elvish code simplifies the language.
This commit does the following:
- Change Equal, Hash, Kind and Repr to treat struct maps like maps.
- Change Assoc and Dissoc to "promote" struct maps to maps.
- Remove the custom Repr method of parse.Source.
- Update documentation to reflect this change.
