Add -Wconversion to clang builds

MSVC has strong opinions on implicit conversions as used in CI, while Clang both
locally and in CI has weaker opinions.  In an effort to avoid subsequent
roundtrips through CI, make clang more strict.  Adding -Wconversion definitely
increases the strength of clang's opinions, apparently to include frowning on
some that even MSVC considers OK, so go make explicit the current implicit
behavior.

src/mem/alloc.h

@@ -369,7 +369,7 @@ namespace snmalloc
         error("Not allocated by this allocator");
       }
 
-      large_dealloc(p, 1ULL << size);
+      large_dealloc(p, bits::one_at_bit(size));
 #endif
     }
 
@@ -429,9 +429,9 @@ namespace snmalloc
       if constexpr (location == Start)
         return ss;
       else if constexpr (location == End)
-        return pointer_offset(ss, (1ULL << size) - 1ULL);
+        return pointer_offset(ss, (bits::one_at_bit(size)) - 1);
       else
-        return pointer_offset(ss, 1ULL << size);
+        return pointer_offset(ss, bits::one_at_bit(size));
 #endif
     }
 
@@ -472,7 +472,7 @@ namespace snmalloc
 
       if (likely(size != 0))
       {
-        return 1ULL << size;
+        return bits::one_at_bit(size);
       }
 
       return alloc_size_error();