Improvements for decimal_class: BigDecimal

[notEthan]

I think it is excellent that it is so easy to preserve arbitrary numeric precision parsing JSON with decimal_class: BigDecimal. This currently has a couple of deficiencies, though.

• Not round-trippable: currently JSON.generate will represent BigDecimal as a string. I'm not sure why this is, when it can be encoded perfectly as a number.
• Obscure documentation - it's not mentioned on the ::JSON module or on the default (ext) parser. It is mentioned on the pure and java parsers.

This PR adds documentation where missing, and changes the generation to be a number, as seems much more natural.

I also split json_addition_test.rb to its own run because require 'json/add/bigdecimal' would override BigDecimal#to_json.

[notEthan]

The implementation for c ext is in progress, and I'm not sure about implementing a java version. I wanted to put this up as a draft to solicit feedback while I address those things.

[notEthan]

I added a c ext implementation but I am very inexperienced with c ruby extensions and it could probably be better. e.g. if it were to #include "bigdecimal.h" then generate_json could use klass == rb_cBigDecimal and the generate_json_bigdecimal nan/infinity checks could use VpIsNaN / VpIsInf, but I'm not sure if I can include bigdecimal.h. I will wait and see about feedback before I try to improve it further or even think about java.