[Docs] Add UltiSnips as an alternative way of inserting unicode characters in Vim

See merge request !849

Prepare changelog for release 4.0

See merge request !841

Make* class fixes

See merge request !845

add FAA to atomic_heap

See merge request !844

proofmode: do not treat pure assertions as persistent

See merge request !799

Make sure that `Make` instances are consistent and "constant time".

See merge request !838

add limit_preserving_impl'

See merge request !839

Make BI operational classes consistent: always use `PROP:Type` instead of `bi`.

See merge request !840