Do not merge a token T into a parse state S if S contains
external tokens that can be *followed* by tokens that could
be shadowed by T.
At this point, the only automated test for this logic is via
the bash grammar, in which the `]` token should not be merged
into states in which `_concat` is valid, because `_concat`
can be followed by a `_special_characters` token, and `]`
would shadow `_special_characters`.
While generating the parse table, keep track of which tokens can follow one another.
Then use this information to evaluate token conflicts more precisely. This will
result in a smaller parse table than the previous, overly-conservative approach.
