py/gc: Move away from using mp_uint_t, instead use uintptr_t and size_t.

The GC works with concrete pointers and so the types should reflect this.
diff --git a/py/mpstate.h b/py/mpstate.h
index c1c1fae..78f497e 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -52,8 +52,8 @@
     #if MICROPY_ENABLE_FINALISER
     byte *gc_finaliser_table_start;
     #endif
-    mp_uint_t *gc_pool_start;
-    mp_uint_t *gc_pool_end;
+    byte *gc_pool_start;
+    byte *gc_pool_end;
 
     int gc_stack_overflow;
     mp_uint_t gc_stack[MICROPY_ALLOC_GC_STACK_SIZE];