fix(frontends/let): let-expression pretty printer

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(build): invoke linja from lean build

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/dependencies): do not stop computing dependencies at error, compute as many as possible, and sign error in the end

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/server): add 'OPTIONS' command to 'lean --server'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/dependencies): send all missing files to standard error, closes #111

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(shell): remove --permissive option, closes #107

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
lean-mode checkpoint
fix up linja