Drastically improve the performance of undo ops grouping on large states

Continuous typing causes the undo states to potentially become extremely
large. The naive assumption that some of the arrays in use would stay
small causes immense slow-downs when determining whether to create a new
undo state.

These bottlenecks have now been removed, and the search depth of various
grouping conditions have been limited to psmaller sizes.
Merge pull request #324 from peitschie/undo-perf-tweaks

Improve the performance of undo ops grouping on large states