feat(frontends/lean/parser): compact definitions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): use LEAN_PATH in the Import command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/cast): use .lean file instead of .cpp file to define casting library

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