compiler: optimise top level operations transformer #1055

The top level operations transformer currently traverses the whole AST looking for call expressions, and tries to decide if they're top level in scope.

But this can be expensive in large ASTs.

Maybe a better approach is:

• Only visit the Program node
• Iterate over the children of Program
• Identify any ExpressionStatements with a CallExpression inside
• Those are your top-level ops

This is a shallow top-down traversal and it's got to be a lot cheaper