Add built-in function for parsing resource strings

[lucperkins]

I was writing some Rego policies for Kubernetes Admission Control objects the other day and wanted to create some example rules regarding resource usage. One barrier that I ran into is that it's quite difficult in Rego to work with strings like 100Mi, 5gb, and 100KB (i.e. the system described here). A built-in function converting those strings into an integer number of bytes would make it dramatically easier to, for example, make comparisons, assign limits, etc. Here's an example:

deny {
    ram_usage > numbytes("5GB")
    disk_usage > numbytes("500MB")
}

I have a PR basically ready to go for this if it sounds useful to others.

[tsandall]

@lucperkins and I discussed this offline. This would be fairly useful for writing k8s admission control policies. In fact, @timothyhinrichs put together a video where he implemented conversions inside of Rego: https://www.youtube.com/watch?v=iBHf7Nzf-Lg. As long as we construct the ast.Number value using math/big operations we won't have to worry about loss of precision.