feat(kernel/inductive): add APIs for retrieving information about declared inductive datatypes, intro rules, and elimination rules

The new API is important for implementing environment transformation procedures, and export Lean environment to different systems (e.g., Coq).

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/inductive): add APIs for retrieving information about declared inductive datatypes, intro rules, and elimination rules