Start getting module loading working.

Right now, it uses a weird "import_" method on String which
should either be replaced or at least hidden behind some syntax.

But it does roughly the right thing. Still lots of corner cases to
clean up and stuff to fix. In particular:

- Need to handle compilation errors in imported modules.
- Need to implicitly import all core and IO types into imported module.
- Need to handle circular imports.
  (Just need to give entry module the right name for this to work.)

include/wren.h

@@ -25,8 +25,12 @@ typedef struct WrenVM WrenVM;
 //   [oldSize] will be zero. It should return NULL.
 typedef void* (*WrenReallocateFn)(void* memory, size_t oldSize, size_t newSize);
 
+// A function callable from Wren code, but implemented in C.
 typedef void (*WrenForeignMethodFn)(WrenVM* vm);
 
+// Loads and returns the source code for the module [name].
+typedef char* (*WrenLoadModuleFn)(WrenVM* vm, const char* name);
+
 typedef struct
 {
   // The callback Wren will use to allocate, reallocate, and deallocate memory.
@@ -34,6 +38,23 @@ typedef struct
   // If `NULL`, defaults to a built-in function that uses `realloc` and `free`.
   WrenReallocateFn reallocateFn;
 
+  // The callback Wren uses to load a module.
+  //
+  // Since Wren does not talk directly to the file system, it relies on the
+  // embedder to phyisically locate and read the source code for a module. The
+  // first time an import appears, Wren will call this and pass in the name of
+  // the module being imported. The VM should return the soure code for that
+  // module. Memory for the source should be allocated using [reallocateFn] and
+  // Wren will take ownership over it.
+  //
+  // This will only be called once for any given module name. Wren caches the
+  // result internally so subsequent imports of the same module will use the
+  // previous source and not call this.
+  //
+  // If a module with the given name could not be found by the embedder, it
+  // should return NULL and Wren will report that as a runtime error.
+  WrenLoadModuleFn loadModuleFn;
+
   // The number of bytes Wren will allocate before triggering the first garbage
   // collection.
   //