Improve token representation in AST

[kortschak]

This change includes vendor directory addition of go-cmp. Unfortunately the go-cmp checks require an end-run around test package hermeticity, but I have made this as minimal as possible.

Please take a look.

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[TristonianJones]

/gcbrun

[kortschak]

Fixed vendoring issue.

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun