doc(kernel/inductive): improve module documentation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/environment): bug in get_extension

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/inductive): store computational rules in an environment extension

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

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/inductive): add normalizer extension for inductive datatypes, add procedure for creating an standard (empty) Lean environment

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