cue: support type conversions

[mpvl]

This is a tracking issue for the implementation of the commented out sections in the spec for conversions.

For instance:

• convert between the integer types
• between byte lists and bytes
• etc

But also to convert a struct to a specific struct type. The latter is quite handy for various purposes.

[pavel-khritonenko]

Want to calculate Sha1 checksum and then encode it into Base64 string, but I'm unable to do that, because:

cannot use [164,124,236,58,150,40,5,40,196,175,13,142,106,236,181,44,17,99,146,48] (type list) as string|bytes in argument 2 to encoding/base64.Encode

[pavel-khritonenko]

And in that case, the main root that [Size]byte defined as the return type in crypto/* packages is not being treated as bytes.

[mpvl]

It still makes sense to support conversions, but it probably makes sense to interpret [Size]byte as []byte, and not a list of ints in the first place.

This would be a relatively minor, but breaking change, but presumably a welcome one and I think it is worth it. Feedback welcome.

[pavel-khritonenko]

@mpvl I wondering if there is a working example of crypto/sha1 pkg usage. Cannot imagine where I would like to have [...int] type instead of bytes. I checked all other public packages, only crypto/* packages have such return type.

And it's a definitely different issue than conversion between types.

[mpvl]

I think it is unlikely. We are still in a change where changes can be made if it makes sense (with due consideration of course), but here I think it is obvious.

These APIs were generated before CUE had a bytes type. Now it does, it seems consistent to use it not only for []byte, but also [...]byte.

[mpvl]

I've checked, and such a change in interpretation would only affect the crypto packages.

[cueckoo]

This issue has been migrated to cue-lang/cue#3.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.