Json Marshall encoder: Add ability to handle rune based keys (single char, fixed length, etc.)

[novicearts]

Is your feature request related to a problem? Please describe.
I'm currently working on a text/font app that requires potentially processing multiple languages, as such I opted to utilize runes as my map keys. For instance I have maps like:

character_table: map[rune][2]int
ligature_attrib_table: map[[2]rune] struct{...}

When I try to feed such structures through to the json.marshal function, I receive the Unsupported_Type error message. Upon looking at the source code, it seems like presently runes are not a supported key type for marshalling

Describe the solution you'd like
It seems like the main fix is to update the json marshal function to include support for rune types (and in general fixed arrays of simple data types if those aren't included, but that might be a separate issue).