Use safeframe under the hood

[Bisaloo]

This PR starts using safeframe under the hood, with an emphasis on minimizing breaking changes and making this switch as invisible as possible to the user.

Some decisions to resolve:

• tags() has been switched to use var = label in place of the previous label = var to help with evaluate the usage of tag_names as arguments in make_linelist using autocomplete #144. However, this means that tags(x, show_null = TRUE) is no longer closely aligned with tags_defaults()
• validate_linelist() now errors when all default tags are not present (validate_linelist() does not error when default tags are not all present #155). Is this what we want?
• some dplyr compatibility seems to have been lost
• should we follow safeframe naming of labels vs tags? The implementation has changed but the features are still the same as before so do we need to change the name?

TODO:

• Add gradual deprecation check
• Add NEWS

Supersedes #150