Maintain writer.js

[saschanaz]

Fixes #109

This PR:

• converts old ES5 syntax to ES2015+
• add tests (by writing IDLs from existing JSON AST, reparsing them and comparing them to the original ASTs)
• fix bugs and support new types to pass the tests

PS: Note that this does not test whitespaces yet as it's not fully understood yet (#97). It will be done on a later PR.

[marcoscaceres]

Glad this is gone