Remove the alamatic.interpreter module.

This will probably come back in a new form soon, but this model of
executing the AST is now defunct so we're going to start from a clean
slate and implement a simpler interpreter that works in terms of the
intermediate language instead.
Simplify the testutil AST nodes.

Now that the AST doesn't get executed, fewer different sorts of dummy
node are needed since these only need to support the tests for generating
intermediate code from an AST.
Test immediate code generation for AssignExpr.