* Rename let-funcs.cpp to LetMgr.cpp

* Rename let-funcs.cpp to LetMgr.cpp
* Move the letMgr into the ParserInterface class. This means it's not in scope until the program exits, instead just during parsing.
* The smt-lib parser makes all its calls into STP via the ParserInterface class.
* Updated the smt-lib parser to use the typing node factory. Explicit calls to BVTypeCheck() are not longer required so have been removed.
* Removed some redundant type checks from the smt-lib parser.
* Updated the cvc parser to use the letmgr in the ParserInterface object.

git-svn-id: https://stp-fast-prover.svn.sourceforge.net/svnroot/stp-fast-prover/trunk/stp@662 e59a4935-1847-0410-ae03-e826735625c1