feat(frontends/lean): relax restricitions on parsing applications of functions containing implicit arguments

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(shell): add 'file not found' error message

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): simplify how implicit parameters are marked

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): relax compatible_denotation predicate

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/operator_info): add << for diagnostic and regular streams

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(tests/lean): include standard error in the expected output

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/io_state): simplify regular/diagnostic

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): implement relaxed operator compatibility in the parser

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