feat: add public C API header wren.h and refactor internal types to WrenVM prefix

include/wren.h

@@ -0,0 +1,39 @@
+#ifndef wren_h
+#define wren_h
+
+#include <stdlib.h>
+
+typedef struct WrenVM WrenVM;
+
+// A generic allocation that handles all explicit memory management used by
+// Wren. It's used like so:
+//
+// - To allocate new memory, [memory] is NULL and [oldSize] is zero.
+//
+// - To attempt to grow an existing allocation, [memory] is the memory,
+//   [oldSize] is its previous size, and [newSize] is the desired size.
+//   It returns [memory] if it was able to grow it in place, or a new pointer
+//   if it had to move it.
+//
+// - To shrink memory, [memory], [oldSize], and [newSize] are the same as above
+//   but it will always return [memory]. If [newSize] is zero, the memory will
+//   be freed and `NULL` will be returned.
+//
+// - To free memory, [newSize] will be zero.
+typedef void* (*WrenReallocateFn)(void* memory, size_t oldSize, size_t newSize);
+
+// Creates a new Wren virtual machine. It allocates memory for the VM itself
+// using [reallocateFn] and then uses that throughout its lifetime to manage
+// memory.
+WrenVM* wrenNewVM(WrenReallocateFn reallocateFn);
+
+// Disposes of all resources is use by [vm], which was previously created by a
+// call to [wrenNewVM].
+void wrenFreeVM(WrenVM* vm);
+
+// Runs [source], a string of Wren source code in a new fiber in [vm]. Returns
+// zero if successful.
+// TODO(bob): Define error codes.
+int wrenInterpret(WrenVM* vm, const char* source);
+
+#endif