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

small issues in the ChainProcessor system. Some tests ending in .BAK
should be runnable but can't for now because of a memory leak somewhere
in the framework.