1.58 - Book release drum beat

Various tuning adjustments, and one feature addition in this version.

Feature:
- coerce_index now works with multiple columns. See ?coerce_index for
details.

Tuning:
- simplehist much more efficient now
- add stancode method for stanfit objects
- map2stan uses rstan's own multicore code now, instead of my
implementation. Should be equivalent, but produces less Terminal spam
now.
- Various documentation edits.