refactor(kernel): the type in let-exprs is not optional anymore, if the user does not provide it, we use a metavariable

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel): normalizer

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel): remove unnecessary files

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