types: Add initial support for varargs

This commit adds initial support for varargs on function
declarations. Varargs are only supported on void functions for
now. In the future, varargs can be supported on any function, but care
will have to be take in the compiler to handle calls that make use of
the legacy calling convention for functions where the output is
captured in the argument list (e.g., plus(1,2,3)).

Signed-off-by: Torin Sandall <torinsandall@gmail.com>