refactor(library/standard): move cast and heq to separate file

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add piext axiom, and theorems that follow from it

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(frontends/lean/elaborator): replace abstract with abstract_local

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