Simplified atom_table.

Previously we had a separate pre-filled atom_table with PDF symbols
which would have been used to initialize the atom_table contained in
each PDF parser object.
Now each atom_table shares an immutable static unordered_map which
contains all PDF symbols. A separate mutable non-shared unordered_map
contains all other symbols. I expect that most of the lookups will be
from the shared unordered_map.