A bitmask where each bit refers to the byte with the same index. If the bit is true, the byte
is initialized. If it is false the byte is uninitialized.
The actual bits are only materialized when needed, and we try to keep this data lazy as long as
possible. Currently, if all the blocks have the same value, then the mask represents either a
fully initialized or fully uninitialized const allocation, so we can only store that single
value.