chore(frontends/lean): rename setoption and setopaque commands to set::option and set::opaque

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

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(builtin/Nat): name convention

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(builtin/Nat): name convention