fix(lean): fix warnings produced by cppcheck

Fix (relevant) warnings produced by http://cppcheck.sourceforge.net.
Most warnings produced were incorrect. The tool does not seem to support some of the C++11 new features.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
checkpoint
checkpoint2
checkpoint3: implement app_rewriter
fix(kernel/expr): add expr operator() with 8 args
feat(library/rewriter): add {type,body,value} rewriter for {lambda,pi,let}
checkpoint4