Improve the JSON serialization for some AST elements

[turbolent]

Work towards #210

Start improving the JSON serialization for some AST nodes:

• Access
• Argument
• ConditionKind
• BoolExpression
• NilExpression

[benjaminkvm]

Curious why you define this type versus just use *BoolExpression below?

[turbolent]

The function tries to add some more properties to the resulting JSON. It could do that by declaring another struct type which has exactly all the same fields like the original, but that would cause a lot of duplication (and maintenance cost).

Marshaling a wrapper struct which embeds the original won't work: the json.Marshal call would in turn call this very same function again for the embedded original, leading to a stack overflow.
This can be prevented by introducing a type alias, which has the same fields as the original, but the MarshallJSON function is not called on it.

For more, see e.g. http://choly.ca/post/go-json-marshalling/