std.data.json formal review

[Sönke Ludwig via Digitalmars-d]

I've added some changes in the latest version (docs updated):

- Switched to TaggedAlgebraic with full static operator forwarding
- Removed toPrettyJSON (now the default), added GeneratorOptions.compact
- The bigInt field in JSONValue is now stored as a pointer
- Removed is(String/Integral)InputRange helper functions
- Added opt2() [1] as an alternative candidate to opt() [2] with a more 
natural syntax

The possible optimization to store the type tag in unused parts of the 
data fields could be implemented later directly in TaggedAlgebraic.

[1]: http://s-ludwig.github.io/std_data_json/stdx/data/json/value/opt2.html
[2]: http://s-ludwig.github.io/std_data_json/stdx/data/json/value/opt.html