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- dependent transformations (ones that depend on cloning history
  and/or metaproperties) have now the same type Trans.trans and
  can be registered via Trans, too.
- load_driver accumulates appropriate tdecls to be appended to
  tasks before transformation/printing.

At this moment, we have almost everything in place and are ready
to remove Register module (subsumed by Trans and Printer in core/)
and Prover module (its functions will move to Driver), and convert
all printers and transformations to use the new infrastructure. 

Not implemented yet:
- appending driver-imposed tdecls to tasks - when and where?
- metas-options - what is the best way to implement them?
- syntax/typing for metas in theories and drivers.

completion : complete theories and goals

(thus, e.g., both alt-ergo and smt printers can register "AC")

installation (work in progress, currently for developers only); why.conf is also searched for in the installation path (Config.why_libdir); in why.conf, theories and drivers filenames are relative to why.conf dirname (when relative)

- 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.