Use usize for transaction-position-in-block values

Use usize for transaction-position-in-block values

We use them largely as indexes into a Vec<Transaction> so there's
little reason for them to be u32s. Instead, use them as usize
everywhere.

We also take this opportunity to add range checks before
short_channel_id calculation, as we could otherwise end up with a
bogus short_channel_id due to an output index out of range.