Initial basic support for CUE

[hairyhenderson]

Fixes #1601

This adds CUE support to gomplate as a native datasource format, as well as data.CUE/cue and data.ToCUE/toCUE functions to allow inline parsing and rendering.

CUE has support for modules and packages (ability to import data from other files), which doesn't work just yet. Key to this is providing a filename, which is awkward with the current parseData signature.

I'll keep this in draft until I'm happy with a solution for this (or until I decide to simply not support importing).

At least to start, CUE support will be single-file only, with no support for importing local packages.

[hairyhenderson]

Update on importing packages from the local filesystem: I may just be missing something, but I just don't think it's going to be possible. Even with providing a filename and import path at parse-time, imports fail.

So at least initially, CUE support will be partial and not support imports (importing builtins does work, though).

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