This allows you to use these functions in modules
that you would like to instantiate multiple times.
Without this your module will be uniquified by
these functions.

AsyncQueue is still a Queue.

RegMapper is fundamentaly DecoupledIO.
Let the user take advantage of this.
Clarify that rules on data handling.

If a device changes a register while it's being read but not yet accepted,
this an lead to 'data' changing while 'valid' is high. A violation. The
problem is that RegMapper is fundamentally DecoupledIO. So fix it with a
Queue.

CSRs that have an effect on I/O should use an explicit FENCE.

The latter feature is by default disabled, since there aren't enough
ID bits.

clint & plic: Move the default addresses

* Suggest sane names for common objects frequently instantiated with factory methods

* Suggest names for common primitives using more Scala-esque Options

Tl2 isolation

Change the configuration interface of SlowIO

Tl2 async nodes

This lets us create nodes which transform from one bus to another.

Move a bunch more things into util package

A lot of utility code was just being imported willy-nilly from one
package to another. This moves the common code into util to make things
more sensible. The code moved were

 * The AsyncQueue and AsyncDecoupledCrossing from junctions.
 * All of the code in rocket's util.scala
 * The BlackBox asynchronous reset registers from uncore.tilelink2
 * The implicit definitions from uncore.util

Setting it to None was a mistake.  It makes it far harder to
diagnose boot bugs, as you end up fetching from random addreses
after trapping.