Add canonicalize ast pass to jsonnetfmt.

[netomi]

This PR is a POC implementation for #721 .

It is intended to get feedback if something like this makes sense and if the implementation of that is reasonable.

The PR only considers objects and arrays and will sort them using the following criteria.

objects:

• locals and asserts are untouched and should be above all other object fields
• object fields are converted to string representation using unparser
• comments and whitespace are stripped when converting to string representation

arrays:

• elements are converted to string representation using unparser