Support customer transformation by providing a textual transformation rule

[xusheng6]

We have several built-in transformations that are very useful, but from time to time we need custom transformations beyond those. We do support custom transformations, however, many of these are only useful maybe for once, so it can be inconvenient to write a plugin for it

I propose that we support a new type of transformation that takes e.g., a byte, and calculate the output based on a rule, and then generate the output. Suppose someone provide the rule as "byte - 0x10 ^ 0x45", then the transformation will calculate each byte accordingly

This is inspired by https://github.com/DidierStevens/DidierStevensSuite/blob/master/translate.py