Rename get_kind() -> kind()

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add expressions (dependent type theory)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add tests for kernel expression

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Update the build image in README.md
Add format.cpp/.h
Update format.cpp