vtable and vtable size depends only on `Table#bb_pos` but calculated in
`Table#_offset` method on each field lookup.
Doing this with every call of `Table#__offset` is redundant.
These values can be read once with change of `Table#bb_pos` and reused
for any field lookup.
A Namespace object is now guaranteed unique.
This cleaned up some old workarounds and latent bugs.
Change-Id: Ic3f12d89947871b03b2c449ba51b3186f953adde
Tested: on Linux.
Bug: 21336857