Moving code of tactic interpretation from Tacenv to Vernacentries.

This allows to directly register globtactics in the Tacenv API, without
having to resort to any internalization function.
Getting rid of atomic tactics in Tacenv.

ML tactics that may be used as simple identifiers are now declared as
a true Ltac entry pertaining to the module that contains the Declare ML
Module statement.
Code cleaning in Tacintern.
Coqide prints succesive hyps of the same type on 1 line

This revert somehow f5d7b2b1eda550f5bf0965286d449112acbbadde about "Hypotheses
don't respect Barendregt convention".
In evarconv, do first-order unification of LetIn's properly, ensuring we compare bodies
of convertible types! Bug found by B. Ziliani.
Moving the decompose tactic out of the AST.
Removing the 'inductive' parameter from tacexpr AST.

It was actually useless, because its only use was in the moved away
decompose tactic.