Properly implement `/=` and `//=`

[Soupstraw]

These operators are supposed to extend the definition with the values on the right hand side.

See https://www.rfc-editor.org/rfc/rfc8610#section-2.2.2

[nc6]

In the sense of parsing CDDL, they are already implemented correctly. See https://github.com/input-output-hk/cuddle/blob/master/src/Codec/CBOR/Cuddle/CDDL/Resolve.hs#L94

What's not implemented is generating CDDL using /= or //= with huddle, since we would generally prefer to alter things at the Haskell level rather than in the CDDL.