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

Since its already incorrect no encoding for simplify, and also no trigger because its buggy inside simplify

- everything is converted to the new shiny way of doing things.
  Well, everything except Gappa, which seems very unifinished anyway,
  and Encoding_instantiate, which is too complex and would like to 
  update it with François.

Also, I commented a little piece of exception reporting in manager/,
will see it with Claude.

THIS IS STILL A WORK IN PROGRESS!
Please inform me about any bugs, ugly APIs, and proposed corrections.

All the non-implemented things, mentioned in the previous commit
message are still in the TODO list and will be done soon.

- 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)