Stabilize vtable api

[ipoupaille]

The use case is for example use Bytes create with memory from mmap.
A file is mapped in memory before construct of the first bytes, and unmapped when the last is dropped.

[NobodyXu]

IMHO marking the all fields of Vtable struct as pub might be a compatibility issue in the future.
What if we want to add a new optional function to Vtable?

I think it's better to have a VtableBuilder to build Vtable where new optional functions can be skipped.