refactor(library/logic/connectives): mark theorems used to prove 'decidable' theorems as transparent

if we don't this, then 'if-then-else' expressions depending on theorems
such as 'and_decidable', 'or_decidable' will not compute inside the kernel
refactor(library/data/nat): prove auxiliary theorems about < and sub asap for the definitional package
feat(library/logic/if): add dependent if-then-else: dite
test(tests/lean/run/div_wf): cleanup div based on well founded recursion
checkpoint

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