This PR improves performance of LRAT trimming in bv_decide.
The underlying idea is taken from LRAT trimming as implemented in
[`lrat-trim`](https://github.com/arminbiere/lrat-trim/t): As we only
filter about half to two thirds of the LRAT proof steps anyway, there is
no need to use tree or hash maps to store information about them and we
can instead use arrays indexed by the proof step directly. This does not
meaningfully increase the amount of memory required but makes the
trimming step basically disappear from profiles, e.g.
`smt/non-incremental/QF_BV/20210312-Bouvier/vlsat3_a72.smt2` [used
to](https://share.firefox.dev/41kJTle) have 8% of its time spent in
trimming [now](https://share.firefox.dev/3QAKI4w) 1.5%.
c1e76e8976