DRISL: BigNum support

[darobin]

We currently support no tag other than 42. The question has come up as to whether we should support BigNum or not.

It is relatively easy to add this at this point without much breakage, it'll be harder later. But it's worth gauging if that just increases complexity too much for current implementers.

[vmx]

A workaround (which I'd prefer) to have BigNum support is using bytes and make sure that your application "knows" (e.g. through a schema or simply code) that it should be interpreted as a BigNum. This would then be kind of similar how ATProto deals with e.g. datetime. It's a string on the CBOR level, but on a layer on top (Lexicons) it gets the meaning of a date.