refactor(kernel): remove semantic attachments from the kernel

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(builtin/heq): merge cast and heq modules

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(doc/lean/tutorial): expand the tutorial

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(README): add link to tutorial in the main page

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(README): add link to tutorial in the main page