Better Documentation of ChiselUtil

[ben-k]

There is lots of useful stuff in ChiselUtil, but it's really easy to not know that this exists, much less how to use it. This useful library of modules should be promoted more prominently in our documentation, and should be documented more thoroughly, including an explanation of the intended use of each parameter in the module instantiation and usage examples. There is marginal documentation of some functions at the end of the Chisel manual, but it is woefully out-of-date.

And please don't tell me that "the code is the documentation"...