RFC: std.json sucessor

[Sönke Ludwig via Digitalmars-d]

I've added support (compile time option [1]) for long and BigInt in the 
lexer (and parser), see [2]. JSONValue currently still only stores 
double for numbers. There are two options for extending JSONValue:

1. Add long and BigInt to the set of supported types for JSONValue. This 
preserves all features of Algebraic and would later still allow 
transparent conversion to other similar value types (e.g. BSONValue). On 
the other hand it would be necessary to always check the actual type 
before accessing a number, or the Algebraic would throw.

2. Instead of double, store a JSONNumber in the Algebraic. This enables 
all the transparent conversions of JSONNumber and would thus be more 
convenient, but blocks the way for possible automatic conversions in the 
future.

I'm leaning towards 1, because allowing generic conversion between 
different JSONValue-like types was one of my prime goals for the new module.

[1]: 
http://s-ludwig.github.io/std_data_json/stdx/data/json/lexer/LexOptions.html
[2]: 
http://s-ludwig.github.io/std_data_json/stdx/data/json/lexer/JSONNumber.html