We need this in order to be able to assemble more fine grained host
function sets.

E.g. we don't want to use `SubstrateHostFunctions` for PVF. We would
better whitelist certain host functions. However, we cannot do that
because we cannot refer to the `Allocator` runtime interface.

I have not been able to find the reason why it wasn't made `pub` in the
first place, but do not see any reason why not to.