Update .travis.osx.yml and add scripts to run doxygen and testcov on travis-ci(OSX)
Pretty print let expressions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add parse_let

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix bug in parser.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>