Nik Everett f5c8c746e6 Implement toString in painless's AST
This should make debugging painless' analysis and code generation a
little easier.

The `toString` implementations mirror the AST somewhat, and look like
`(SSource (SReturn (ENumeric 1)))`.
2016-11-21 16:24:10 -05:00
..
2016-11-10 09:59:13 -08:00