chore(frontends/lean/scanner): remove dead code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(frontends/lean/scanner): missing tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): parser aborted if the scanner throws an exception in the first call to scan(); position information was being shown twice for scanner exceptions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/scanner): assertion violation, and add more tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean): kernel exception pp method

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(frontends/lean/frontend): remove dead code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean): alias command error

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