Merge pull request #38 from BarAgent/statements

Pratt-style expression parse tree.
Remove unused properties from new file actions.
Fix definitions of constants and variables.
Make body & constituents non-private.

These should not be private as without that, we lose ordering and
operating on the PSI structure for things like refactorings and
surrounders becomes much more difficult / error prone.
Rename file icons, add Retina versions.

Thanks Seyi for the Retina versions!
Fix definitions of slots
Define unique strings as tokens

Fix #40
Remove import of removed class.
Support method separators.
Next / Previous method support.
Add testworks definers.
Add folding for tests and suites.
Add icons for tests and suites.
Shouldn't prepend 'module' to the name.
Update changelist.
Make statements extend DylanStatement.
Make operand private. Move usages of operand to operand_expr.
Make leaf private.
Allow test & suite as variable names again.
Fix up ordinary_name being private.

We introduce ordinary_binding_name for the places that want to
use ordinary_name, so that we can keep it private so that it doesn't
get exposed via variable_name.