copilot-bluespec: Update DESIGN.md to reflect new input/output order. Refs #677.

The current Bluespec backend leads to Verilog code that requires manual
manipulations in order to work correctly. Specifically, Copilot externs, which
are inputs to the Copilot monitoring system, are treated as _outputs_ in
Verilog, and Copilot triggers, which can be considered outputs of the
monitoring system, are treated as _inputs_. This is the opposite of the
interface that we would like users to work with, and of what other backends
generate (e.g., C99).

Now that `copilot-bluespec` has been updated to invert the order in which
Verilog inputs and outputs are declared, this updates the prose in the
`DESIGN.md` document to reflect the new design. The new design is just
different enough from the old one that it made sense to completely rewrite
certain portions of the document. One of the more notable changes is that there
are now separate sections about running the generated Verilog code on hardware
versus simulating the generated code in software, as these are distinct use
cases that require separate discussion.

Author: RyanGlScott