This PR improves the error messages produced by the `split` tactic,
including suggesting syntax fixes and related tactics with which it
might be confused.
Note that, to avoid clashing with the new error message styling
conventions used in these messages, this PR also updates the formatting
of the message produced by `throwTacticEx`.
Closes#6224
On issue #906, `simp` spends a lot of time checking the cache. This
process is time consuming and doesn't allocate memory. Before this
commit, it would take a long time to get the "maximum number of
heartbeats" error message on the example included in this issue.
Closes#906
