gotypes: fix argument size for signatures without return types (#212)

This fixes a bug in argument size calculation in the case where the function
has no return values. Previously it was padding the argument struct to max
alignment, but this only happens if there are return values following.

Updates #191

gotypes/signature_test.go

@@ -159,6 +159,7 @@ func TestSignatureSizes(t *testing.T) {
 		{"func(uint64) uint64", 16},
 		{"func([7]byte) byte", 9},
 		{"func(uint64, uint64) (uint64, uint64)", 32},
+		{"func(uint16)", 2},
 	}
 	for _, c := range cases {
 		s, err := ParseSignature(c.Expr)