Refactor use of sizeclasses (#415)

The primary aim for this refactor is to use a representation for
sizeclasses that uniformly covers both large and small.  This allows
certain operations such as alloc_size and external_pointer to be
uniformly implemented.

The additional types make clear which kind of sizeclass is in use.

This also tidies up the code for sizeclass based divisible by and
modulus.

It fixes a bug in rust_realloc that didn't correctly determine a realloc
was required for large classes.

src/test/func/first_operation/first_operation.cc

@@ -166,7 +166,7 @@ int main(int, char**)
   f(5);
   f(7);
   printf("\n");
-  for (size_t exp = 1; exp < snmalloc::MAX_SIZECLASS_BITS; exp++)
+  for (size_t exp = 1; exp < snmalloc::MAX_SMALL_SIZECLASS_BITS; exp++)
   {
     auto shifted = [exp](size_t v) { return v << exp; };