Use tag versions to implement fallback tags.

Use tag versions to implement fallback tags.

This commit adds new data structure to represent copy commands for
tags: list of pairs (X, Y) which stand for command 'X = Y'.

Previously we used the same bit vectors that are used for constructing
DFA states, but this representation is inconvenient.

We no longer need special tag names (postfixed with underscore) for
fallback tags: we simply use a fresh version for each fallback tag.