xcvm: merge router and gateway together

[mina86]

The two contracts are tightly coupled and implicitly trust each other.
There’s very little gained by having them separated and merging them
simplifies development and deployment so do that.

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[mina86]

With #3693 merged this needs a minor fix before merging. I’ll update this PR in a moment.

[mina86]

@dzmitry-lahoda, PTAL