feat(frontends/lean/elaborator): generate type information for placeholders

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/num): avoid exception

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/pp): pretty print numerals

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add information about synthesized placeholders, closes #39

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs/lean-flycheck): override flycheck-try-parse-error-with-pattern

For lean-mode, we override flycheck-try-parse-error-with-pattern to
increase column number by 1. It uses defadvice.