typo in generic type instantiation crashes the compiler

[Thierry Delisle]

This code crashes the compiler because of "typet_t" instead of "type_t"

forall(dtype T | sized(T))
struct wrapper{
	T * ptr;
	int size;
};
typedef long long int type_t;
struct outer {
	wrapper(typet_t) w;
};

[Rob Schluntz <rschlunt@…>]

In 48fa824:

Add error check for unimplemented expression parameters for generic types [fixes #58]