std.json improvements

[daurnimator]

Follow-ups now that #4304 was merged.

• optional whitespace options when serialising to json
• use jsonStringify trait on json.Value objects
• code deduplication
• more tests

[andrewrk]

The "json" part of this function name is redundant. The full path to this function is:

std.json.Value.jsonStringify

but it would be better to be

std.json.Value.stringify

[andrewrk]

Also this function appears to be redundant with std.json.WriteStream().emitJson

[daurnimator]

The "json" part of this function name is redundant. The full path to this function is:

std.json.Value.jsonStringify

but it would be better to be

std.json.Value.stringify

This function is a trait.

Also this function appears to be redundant with std.json.WriteStream().emitJson

Yes: I have a followup PR to make that function use this one.

[JustinRyanH]

This function is a trait.

To make things clear you are expecting that you can

pub fn foo(val: var) []u8 {
   return val.jsonStringify()
}

and it will for any enum, struct, or whatever that has a jsonStingify method

[daurnimator]

@JustinRyanH yes. and if you implement jsonStringify on your own struct/union/enum then std.json.stringify uses that instead of the "normal" routine for that data type.

[andrewrk]

I think this is backwards, std.json.stringify should be utilizing std.json.WriteStream rather than the other way around.