- Implement the Leon Library in Leon-land rather than Scala-land.
  import leon.Utils._ becomes import leon.lang._
  import leon.Annotations._ becomes import leon.annontation._

  For now, the library defines generic Options and Lists.
  The library is automatically imported from the ./leon script, unless
  the --library=no option is passed.

- Support parsing of multiple files and modules.

- Introduce new annontations:
    @ignore: remove definition from Leon
    @verified: do not consider for verification unless explicitly
        specified

Use a datagen-based solver to find simple counter-examples. Note that
this solver returns Unknown most of the time, so it is best to combine
it with a full-fledged solver.

PortfolioSolver allows us to combine solvers and have them run in
parallel. The first result (!= Unknown) is used. Solvers can be selected
for verification using the --solvers option.

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.

Improve debugging capabilities of Pretty/Scala printer w.r.t. types

- 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.

Refactor the TimeoutSolver and TimeoutAssumptionSolver so
that they no longer use innerCheck methods. We now only
create solver as subclassing Solver or IncrementalSolver and
implementing the interuptible trait, and we can turn them into
TimeoutSolver at instantiation time with a mixin.

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

Previously, array access were generated via a code transformation in
ArrayTransformation in xlang, where Array Select and Update were
wrapped in if-then-else with an Error in the else branch. They are
now natively supported in VC generation.  Some testcases --- that would
trigger bugs when --xlang is not used with functional array ---- are
included in this commit

The commit introduces a new abstraction to traverse Leon trees and collect
path conditions. This traverser is used for the implementation of the
VC generation for arrays.

- remove superfluous implementations doing almost the same
- introduce pre/post traversal and transformers as foreach/map/fold
- Redefine other operations exists, contains, as folds
- Fix freshenLocals bug found by Ravi

- Simplify code generation by replacing CompilationEnvironment with a
  simple scope state.

- Support Choose construct in both evaluators.

- Introduce RecursiveEvaluator (renamed from Naive) and TracingEvaluator
  which tracks intermediate values as well.

- Introduce offset as well as ranged positions, extract all positions
  from trees. Try to propagate them as much as possible. Introduced
  .copiedFrom

- Remove dead-code, and improve TreeOps a bit.

- Introduce Pretty-printer arguments