The rounding code can be integrated directly into the sizeclass code,
which means, the code paths can be simplified slightly.
The special case for zero mantissa bits is not required, as the compiler
generates the same code from the more complex path due to
eliminating dead code after folding the constant zero through.
Introduce a wrapper that ensure various fields of meta data, represent
valid indexes into the the address space, and not beyond the current
slab/superslab.
If a sizeclass in the metadata is corrupted, then this can be used to
force an index beyond the end of these tables. This extends the tables
to the next power of two, and uses a mask on the index, so they are
always either a valid piece of data, or zero.
