Grammar missing ability to have just a function call. #716

Currently there is no way in the grammar to have a function call by itself on a line, it is not in the list of statements.

For example, the following will fail to parse:

fn a() -> void {
  return;
}

fn b() -> void {
  a();
  return;
}

The statements list needs to be extended to have something like:

 | IDENT paren_rhs_stmt

This means we'll have to do lookahead at in the statement parsing because you can't tell if this is a call or an assignment straight away.