improve [core] docs

See merge request iris/iris!503

Fixes #345

The old code contained a bunch of unused spaghetti that was not cleaned up
after a refactoring.

@jihgfee menaged to trigger a wrong code path in the old code, but I failed
to turn his test case into a self-contained one.

add conversions between functors

Closes #342

See merge request iris/iris!498

add options file for library-wide configuration

Closes #66

See merge request iris/iris!491

add internal_eq_entails uPred law

See merge request iris/iris!499

Document Makefile flags NO_TEST and MAKE_REF together

See merge request iris/iris!500

Use simple variables in Makefile

See merge request iris/iris!501

var=foo is a "recursive variable assignment", which is expanded on every
use (including re-running shell commands). What we really want is
var:=foo, which is expanded at definition time.

Generalize the type of gmap_equivI.

See merge request iris/iris!495

Bump std++

See merge request iris/iris!494

do not eta-expand when sealing

Closes #285

See merge request !490