Don't use module import string when loading imported variables.

This is an interim step towards supporting relative imports. Previously,
the IMPORT_VARIABLE instruction had a constant string operand for the
import string of the module to import the variable from. However, with
relative imports, the import string needs to be resolved by the host
all into a canonical import string. At that point, the original import
string in the source is no longer useful.

This changes that to have IMPORT_VARIABLE access the imported ObjModule
directly. It works in two pieces:

1. When a module is compiled, it ends with an END_MODULE instruction.
   That instruction stores the current ObjModule in vm->lastModule.

2. The IMPORT_VARIABLE instruction uses vm->lastModule as the module to
   load the variable from. Since no interesting code can execute between
   when a module body completes and the subsequent IMPORT_VARIABLE
   statements, we know vm->lastModule will be the one we imported.

test/api/main.c

@@ -91,7 +91,8 @@ static WrenForeignClassMethods bindForeignClass(
   return methods;
 }
 
-static void afterLoad(WrenVM* vm) {
+static void afterLoad(WrenVM* vm)
+{
   if (strstr(testName, "/call.wren") != NULL)
   {
     callRunTests(vm);