ruby: Expose MessageBuffers as SimObjects

Expose MessageBuffers from SLICC controllers as SimObjects that can be
manipulated in Python. This patch has numerous benefits:
1) First and foremost, it exposes MessageBuffers as SimObjects that can be
manipulated in Python code. This allows parameters to be set and checked in
Python code to avoid obfuscating parameters within protocol files. Further, now
as SimObjects, MessageBuffer parameters are printed to config output files as a
way to track parameters across simulations (e.g. buffer sizes)

2) Cleans up special-case code for responseFromMemory buffers, and aligns their
instantiation and use with mandatoryQueue buffers. These two special buffers
are the only MessageBuffers that are exposed to components outside of SLICC
controllers, and they're both slave ends of these buffers. They should be
exposed outside of SLICC in the same way, and this patch does it.

3) Distinguishes buffer-specific parameters from buffer-to-network parameters.
Specifically, buffer size, randomization, ordering, recycle latency, and ports
are all specific to a MessageBuffer, while the virtual network ID and type are
intrinsics of how the buffer is connected to network ports. The former are
specified in the Python object, while the latter are specified in the
controller *.sm files. Unlike buffer-specific parameters, which may need to
change depending on the simulated system structure, buffer-to-network
parameters can be specified statically for most or all different simulated
systems.

ruby: remove message buffer node

This structure's only purpose was to provide a comparison function for
ordering messages in the MessageBuffer. The comparison function is now
being moved to the Message class itself. So we no longer require this
structure.

ruby: move files from ruby/system to ruby/structures

The directory ruby/system is crowded and unorganized. Hence, the files the
hold actual physical structures, are being moved to the directory
ruby/structures. This includes Cache Memory, Directory Memory,
Memory Controller, Wire Buffer, TBE Table, Perfect Cache Memory, Timer Table,
Bank Array.

The directory ruby/systems has the glue code that holds these structures
together.

BuildEnv: Eliminate RUBY as build environment variable
This patch replaces RUBY with PROTOCOL in all the SConscript files as
the environment variable that decides whether or not certain components
of the simulator are compiled.

network: convert links & switches to first class C++ SimObjects

This patch converts links and switches from second class simobjects that were
virtually ignored by the networks (both simple and Garnet) to first class
simobjects that directly correspond to c++ ojbects manipulated by the
topology and network classes. This is especially true for Garnet, where the
links and switches directly correspond to specific C++ objects.

By making this change, many aspects of the Topology class were simplified.

ruby: moved topology to the top network directory

Moved the Topology class to the top network directory because it is shared by
both the simple and Garnet networks.

ruby: Convert most Ruby objects to M5 SimObjects.
The necessary companion conversion of Ruby objects generated by SLICC
are converted to M5 SimObjects in the following patch, so this patch
alone does not compile.
Conversion of Garnet network models is also handled in a separate
patch; that code is temporarily disabled from compiling to allow
testing of interim code.

scons: update SCons files for changes in ruby.