Add some overloads

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Rename/Reorg some kernel files

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

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add has_free_vars/closed function

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