Using `SafeCast` can result in reverts in `Safe` functions

[jalextowle]

Several of the core Hyperdrive flows rely on Safe functions that will proactively catch arithmetic failures and return failure flags instead of reverting. After addressing Spearbit issue #55 in #812, it is now possible for arithmetic failures while casting to result in reversions in flows that we expect to always succeed.

Several changes need to be made to address this:

1. Add a powSafe variant that returns a failure flag instead of reverting when arithmetic issues occur.
2. Add to<X>Safe variants to the SafeCast functions that are currently being used with Safe functions.
3. Ensure that these safe variants are being used within the Safe functions used within Hyperdrive.

[jalextowle]

Closing this since we've addressed the issues relating to this that we're going to fix. There are still cases where we could run into problems with extremely large numbers (on the order of 2^255) but this is very unlikely to happen in practice.