refactor(builtin/kernel): prove eta using function extensionality, and rename abst and abstpi to funext and allext

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(builtin/kernel): prove iff::intro, and add a new name for it boolext (Boolean extensionality)

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