Allow excluding attached names via `!`

[Konrad Rudolph]

Please describe your feature request

Enable the following syntax:

box::use(stats[..., ! filter])

To import and attach all names from the ‘stats’ package, except the name filter. This comes in handy to avoid name conflicts between two attached packages (e.g. stats::filter and dplyr::filter) and makes order of import declarations less important.

Ensure that ! declarations are only valid in conjunction with ... wildcard declarations (since they are otherwise meaningless).