Clean up obsolete comment.
Add a type of untyped term to Ltac's value.

It is meant to avoid intermediary retyping when a term is built in Ltac. See #3218.

The implementation makes a small modification in Constrintern: now the main internalisation function can take an extra substitution from Ltac variables to glob_constr and will apply the substitution during the internalisation.
Untyped term in tactics: add an grammar entry to construct them.

The syntax is uconstr:term.
Untyped terms in tactic: function [type_term c] to give a typed version of [c].
Untyped terms in tactic: add the possibility to use a typed term inside an untyped term.
Allow [uconstr:c] as argument of a tactic.
Small refactoring in Ltac parsing rules.
Document untyped terms in tactics.
CHANGES: untyped terms in tactics