Use a template language for Expression and Statement templates

[woodruffw]

Right now, subclasses of Expression and Statement hardcode their abstract syntax features in the view and concretize methods. To avoid duplication and to make addition of new subclasses easier, these should be lifted to a _template (or similar) class attribute that contains a Handlebar (or other) template.

For example:

class IfStmt(Statement):
    _template = """
    if ({{ cond }}) {
      {{ expr }}
    }
    """

view and concretize could then use _template as appropriate.