Support Better IFC Label Configuration

[dz333]

Currently, FIRRTL has a default implementation of labels, which is the same as the one described in the Hyperflow paper.

These labels (in chisel code) look like this:
Label(Level(<confidentiality representation>), Level(<integrity representation>))

Users may want simpler (or more complex) labels than these, for example those that only check confidentiality. Users may additionally want to modify how labels are represented as runtime and how the "flows to" check is implemented.

The base class is implemented in src/main/firrtl/Policy.scala but it would be nice to have a suite of supported implementation.

Furthermore, it would be nice to add a configuration option to select between any of the default or any "installed" policies. This issue should be split into multiple sub issues.