Add limits to XCMv4 (#3114)

Some more work regarding XCMv4. Two limits from v3 were not transferred
over, those are:
- The instructions limit
- The number of assets limit
Both of these are now in v4.

For some reason `AssetInstance` increased in size, don't know why CI
didn't catch that before.

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: command-bot <>