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.
Also fixed most codegenerators using the wrong namespace when multiple
namespace were used in a file, with some files not being generated.
Change-Id: Ib42969221239d7244e431cbd667ef69200fc415f
Tested: on Linux.
Bug: 29338474