feat(kernel): add trust_imported flag, it skips type checking of 'pre-compiled' Lean modules

"Pre-compiled" .olean files were already type checked. The flag -t instructs to Lean to skip
type checking when importing these files.
TODO: add a check-sum.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel): add trust_imported flag, it skips type checking of 'pre-compiled' Lean modules