feat(util/lean_path): add dirname and path_append aux functions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/module): relative module path

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/dependencies): take relative paths into account when computing dependencies

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/standard): use relative paths in some files in the standard library

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean/run): add unicode in identifiers test

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