Add a way to pad transactions' size if needed by Simplicity cost metric

[schoen]

Simplicity transactions have an additional "cost" associated with the computational resources consumed by verification of the results of executing the contract.

This cost is paid for by ensuring that the transaction has a sufficiently high weight (using conventional weight calculations) to cause the associated fee to be adequate according to the cost formula. Currently, most typical Simplicity programs inherently produce large enough transactions to satisfy this requirement, so there hasn't been a need to artificially increase the size of their transactions in order to get them accepted by Liquid nodes. However, this might not always be the case for more computationally-intensive contracts in the future, e.g. those that use loops or that call expensive jets repeatedly for some reason.

So, we could use a standardized way to pad transactions to make them deliberately larger when this turns out to be required by the cost formula.

@apoelstra suggested that the appropriate place to specify and implement the basis for this is here in rust-elements.

[apoelstra]

We should add functionality to rust-simplicity to obtain the amount of padding needed, and to this library to add this padding as annex data in PSET.