Allow to construct arbitrary tags and extract tag numbers.

[turbocool3r]

I'm currently working on a builder/parser for the Image4 format that is used for boot images on Apple platforms. The format uses 32-bit tags to encode keys of property dictionaries (see "IMG4 Manifest" part of https://www.theiphonewiki.com/wiki/IMG4_File_Format). Here is an example of such a file. These 4-byte tags are called 4CC and are basically arbitrary. There is no public specification for the format, but these look like EXPLICIT tags of the PRIVATE class. I'd like to have a way to extract this information if possible and the best case would be if it's possible to make these tags. Maybe allowing to make arbitrary non-universal tags is the way?

[alex]

I think we likely need to make the Tag constructor public, then it should be possible: https://docs.rs/asn1/latest/asn1/struct.Tag.html