Factor out calldata struct decoding

[haltman-at]

This PR factors out calldata struct decoding into its own function to reduce the amount of copypaste. It contains no functional changes (or at least, it shouldn't).

Notes:

1. I didn't factor out array decoding; I didn't think that really needed it.
2. I didn't include memory struct decoding, even though that also works essentially the same now, because, well, that wouldn't have been so convenient. In any case I think the copypaste, while not gone, is now down to levels I'd consider acceptable.

[gnidan]

Looks good @haltman-at!