add android support (#171)

* adjust for android

* update docs

* add const qualifier to `alloc_size`

* check const qualifier in cmake

src/mem/mediumslab.h

@@ -39,9 +39,10 @@ namespace snmalloc
       return OS_PAGE_SIZE;
     }
 
-    static Mediumslab* get(void* p)
+    static Mediumslab* get(const void* p)
     {
-      return pointer_align_down<SUPERSLAB_SIZE, Mediumslab>(p);
+      return pointer_align_down<SUPERSLAB_SIZE, Mediumslab>(
+        const_cast<void*>(p));
     }
 
     void init(RemoteAllocator* alloc, sizeclass_t sc, size_t rsize)