- accept predicate symbols as functions to bool in programs
- get rid of *_bool comparisons in theories/programs.why

this fixes a problem with encoding_decorate, where
a parsed Prelude theory contained unneeded "use Tuple2"

- bring driver syntax closer to that of theories
- some simple API improvements

- introduce a new Theory.tdecl "Meta" to be used for tags
- simplify cloning procedure, get rid of the th_clone field
- when a goal proposition is discarded during cloning, 
  it's still keeped in the theory as a "skip proposition",
  this is needed to preserve/clone every local identifier.
  Skip propositions are eliminated during task formation.
- get rid of a separate Task.tdecl type
- reorganize the Task.task_hd record:
  * use/clone history is cached in a theory-keyed map;
  * meta-properties are cached in a tagname-keyed map.
  This is done to simplify the fine-grained configuration
  of transformations.

constructor parameters : mainly useful for records, see 
examples/programs/vacid_0_sparse_array

  in the logic language into (more or less) higher-order style.
  For example,

    logic func (x : int, y : int, 'a list) : (int, 'a) map list

    logic pred (bool, int * real)

  is now written:
    
    logic func (x y : int) (list 'a) : list (map int 'a)

    logic pred bool (int, real)

  Note that types use prefix notation (as in Coq) and the types
  of tuples are now written as (type1, type2, ..., typeN).

- Use the same syntax of type expressions in the program language.

- Allow binders of the form (x y:int) in the program language.
  

programs: complete rewrite of the type-checker in progress (in particular, types dterm and dfmla have been moved to Denv, together with some functions); make does not compile whyml anymore; make bench will stop at the first program test; with my apologies if I messed up somewhere...

registered formats: simplification: no more parse_only function, but boolean optional argument instead

Earlier we anticipated declaring projections in this way,
but never implemented it, and I still think this would
be a hack, not in line with the rest of the language.
Anyway, it's a three-line change, easily revertible.

- check pattern well-formedness in Pattern
- add arguments to exceptions in Ty and Term