add android support (#171)

* adjust for android

* update docs

* add const qualifier to `alloc_size`

* check const qualifier in cmake

src/mem/alloc.h

@@ -457,7 +457,7 @@ namespace snmalloc
       return pointer_cast<void>(external_address<location>(p));
     }
 
-    static size_t alloc_size(void* p)
+    static size_t alloc_size(const void* p)
     {
       // This must be called on an external pointer.
       size_t size = ChunkMap::get(address_cast(p));

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)

src/mem/metaslab.h

@@ -140,9 +140,9 @@ namespace snmalloc
       return (slab_end - allocation_start) % size == 0;
     }
 
-    static Slab* get_slab(void* p)
+    static Slab* get_slab(const void* p)
     {
-      return pointer_align_down<SLAB_SIZE, Slab>(p);
+      return pointer_align_down<SLAB_SIZE, Slab>(const_cast<void*>(p));
     }
 
     static bool is_short(Slab* p)

src/mem/superslab.h

@@ -65,9 +65,10 @@ namespace snmalloc
       StatusChange = 2
     };
 
-    static Superslab* get(void* p)
+    static Superslab* get(const void* p)
     {
-      return pointer_align_down<SUPERSLAB_SIZE, Superslab>(p);
+      return pointer_align_down<SUPERSLAB_SIZE, Superslab>(
+        const_cast<void*>(p));
     }
 
     static bool is_short_sizeclass(sizeclass_t sizeclass)