adding bigSubstring and bigLength for strings.

library/lang/StrOps.scala

library/lang/package.scala

library/theories/String.scala

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

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

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

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

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/purescala/Expressions.scala

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

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

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

src/main/scala/leon/solvers/theories/StringEncoder.scala