feat(kernel/formatter): improve simple printer support for Pi and lambda

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/type_checker): memory access violation, closures (for printing error messages) had a uninteded reference to the type_checker

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/list_fn): add get_ith function

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/abstract): add more Fun functions for simplifying the creation lambda-expressions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/inductive): generate computational rules RHS for inductive datatypes

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