Simplification of the tclCHECKINTERRUPT tactic.
Fixing bug #3528.
Fix bug when defining primitive projections mixing defined and abstracts fields.
Add test-suite file. Compute the name for the record binder in the
eta-expanded version of a projection as before.
Fixing commit 50237af2.

Indeed, generalized binders are unnamed, because their name is generated on the
fly.
Not using a "cut" in descend_in_conjunctions induced more success. We
at least remove the successes obtained by trivial unification of a
meta with the goal, so as to avoid surprising results. We generalize
this to variables which will only eventually be replaced by metas.
Simplification of Genarg unpackers.

Instead of having a version of unpackers for each level, we use a dummy argument
to force unification of types.
Type-safe version of genarg list / pair / opt functions.
Define an "apply under binders" tactic
Add an example and explanation of usage

(For [binder apply])