cgo: implement struct types

Not complete: packed structs are treated as regular structs.

testdata/cgo/main.c

@@ -8,6 +8,7 @@ double globalDouble = 3.2;
 _Complex float globalComplexFloat = 4.1+3.3i;
 _Complex double globalComplexDouble = 4.2+3.4i;
 _Complex double globalComplexLongDouble = 4.3+3.5i;
+collection_t globalStruct = {256, -123456, 3.14};
 
 int fortytwo() {
 	return 42;

testdata/cgo/main.go

@@ -36,6 +36,7 @@ func main() {
 	println("complex float:", C.globalComplexFloat)
 	println("complex double:", C.globalComplexDouble)
 	println("complex long double:", C.globalComplexLongDouble)
+	println("struct:", C.globalStruct.s, C.globalStruct.l, C.globalStruct.f)
 }
 
 //export mul

testdata/cgo/main.h

@@ -5,6 +5,12 @@ int doCallback(int a, int b, binop_t cb);
 typedef int * intPointer;
 void store(int value, int *ptr);
 
+typedef struct collection {
+	short s;
+	long  l;
+	float f;
+} collection_t;
+
 // test globals
 extern int global;
 extern _Bool globalBool;
@@ -14,6 +20,7 @@ extern double globalDouble;
 extern _Complex float globalComplexFloat;
 extern _Complex double globalComplexDouble;
 extern _Complex double globalComplexLongDouble;
+extern collection_t globalStruct;
 
 // test duplicate definitions
 int add(int a, int b);