- Implement smt and smt-z3/smt-cvc4 as options for --solvers

- Uses a main solver sources for verification/CLP, taking into account
  --solvers

- Implement Z3-SMT sets

- Add tests for Z3-SMT

Methods:
--------

Methods are now supported in ADT roots only (e.g. single case classes or
abstract classes). The phase `MethodLifting` converts them to normal
global functions which makes them transparent to the rest of Leon.

Introducing Leon Library:
-------------------------

Common structures and functionalities can be shipped as part of the
Library.scala file, which is a set of ADTs with methods.

Imported via `import leon.Library._`

New Annotations:
----------------

- @verified means library function will not be considered for verification
  unless specifically given through --functions

- @proxy allows non-pure bodies, which will be silently ignored.
  pre/post need to be in purescala.

Functions without bodies (e.g. proxy methods) are now materialized with
a choose construct automatically, replicating the post-condition. This
allows compilation to bytecode and execution of VCs refering to proxy
methods.

Tree Modifications:
-------------------
- FunDef now have params, not args.

- VarDecl is renamed ValDef.

- Added methods in ClassDef, as well as This and MethodInvocation,
  both are converted to normal function calls by the MethodLifting
  phase.

- Calling functions are now typed, FunctionInvokation takes a
  TypedFunDef which is basically a FunDef with type parameters' values.
  Instantiation of types within the signature/body is done on demand
  through this wrapper class.

- Operations on ADTs are now taking a *ClassType instead of *ClassDef.
  Similarly, Case/AbstractClassType takes values for type parameters.

- Introduces a GenericValue tree for models targeting abstract types.
  e.g. foo[T](a: T, b: T) { a == b } ensuring (_) will find a model with
  { a -> T#1, b -> T#2 }

- Only "simple" hierarchies allowed with type parameters. All members of
  the hierarchy must define the same number of type parameters and
  correctly pass them to parent classes.

- Type parameters are invariant.

UnrollingSolver now uses an underlying incremental solver
and use push and pop to drive the unrolling instead of creating
a new solver each time

Solvers wrap solvers or factories, depending on the needs.
Factories no longer wrap factories, except for the special case of
timeoutsolverfactories (it does it in a typesafe way though).

Fix TupleRewrite with new posts, fix ScopeSimplified, Fix pretty printer