Only looks at first character when interpreting Ext. Json

Only the first character in field name is examined when interpreting extended JSON. Before, a '$' anywhere in the field name spawned special handling; now it is restricted to just examining the first character.

Proper Extended JSON serialization added.

The serialization procedures now generate proper Extended Json per the spec.

https://docs.mongodb.com/manual/reference/mongodb-extended-json/

• $ is canonical but indented
• toJsonStr is canonical but minimized
• pretty is relaxed and indented

If parsed into proper JsonNode, there is also intepretExtendedJson to move it back to proper BSON.

Added marshal submodule

Added a marshal submodule for converting between object types and Bson.

Initial release

The initial release of the library. Many thanks to https://github.com/SSPkrolik/nimongo as the template for much of this code. This separate library helped make a related project possible.