Add compile error for generic Main

[Theodus]

This PR adds a compile error for a Main actor with type parameters.

closes #1936

[jemc]

What do you think about moving this check to the syntax_entity function in pass/syntax.c? There are some closely related checks there, such as enforcing that Main must be an actor, and enforcing that C API actors do not have type parameters.

[Theodus]

@jemc seems reasonable, I'll try

[Theodus]

@jemc the check has been moved to the syntax pass.

[jemc]

Thanks!

The appveyor CI failure was unrelated, so I'm merging.