Added higher-order function support to Leon

src/main/java/leon/codegen/runtime/FiniteLambda.java

src/main/java/leon/codegen/runtime/Lambda.java

src/main/scala/leon/codegen/CodeGeneration.scala

src/main/scala/leon/codegen/CompilationUnit.scala

src/main/scala/leon/datagen/VanuatooDataGen.scala

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

src/main/scala/leon/frontends/scalac/ASTExtractors.scala

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/purescala/Extractors.scala

src/main/scala/leon/purescala/PrettyPrinter.scala

src/main/scala/leon/purescala/TreeOps.scala

src/main/scala/leon/purescala/Trees.scala

src/main/scala/leon/purescala/TypeTreeOps.scala

src/main/scala/leon/purescala/TypeTrees.scala

src/main/scala/leon/solvers/SolverFactory.scala

...main/scala/leon/solvers/combinators/UnrollingSolver.scala

src/main/scala/leon/solvers/smtlib/SMTLIBCVC4Target.scala

src/main/scala/leon/solvers/smtlib/SMTLIBSolver.scala

src/main/scala/leon/solvers/smtlib/SMTLIBTarget.scala

src/main/scala/leon/solvers/templates/FunctionTemplate.scala