Convert the backend to multi-entry functions

This change is the culmination of:

c02befa Add signal checks to each function entry
2cc42b9 Update the stack limit checker to multi-entry functions
07e1930 Convert exception handlers for multi-entry functions
3351961 Update the chunkifier for multi-entry functions
b63e976 Update stack allocation for multi-entry functions
837c7ec Copy files for the single-entry backend

There is a significant amount of changes in the Backend functor itself:

 *  Profiling remains excluded from the work.  Support for
    profiling will be added back in at a later time (at least after all
    of the optimization passes have been implementd and extra entry
    points can safely be added to functions).

 *  Function calls transfer to function entries, not function labels.
    Function labels shouldn't appear in Machine code.

 *  One function entry block (or prelude) is generated for each function
    entry, so that each entry point does the correct setup for being the
    callee in an intra-procedural transfer.

 *  The MeBackendfunctor is exported to the rest of the compiler.
Add the multi-entry backend to the compilation pipeline

The '-check-multi-entry true' option now does more than just exercise
the multi-entry pipeline: it uses the new pipeline to generate the
programs.  It's now possible to test the multi-entry pipeline with the
regression tests.
Use correct entry point for RSSA main function

When translating the SSA2 program into RSSA, we end up with two main
functions:

1) The logical 'main' function from SSA2
2) A function to store globals, eventually calling the 'main' in (1).

The attempt to update this didn't handle the FuncEntry labels correctly:
it gave the new main (2) the same FuncEntry as the SSA main (1).  Since
the SSA main stuck around, and FuncEntrys become labels in the Machine
IL we ended up with duplicate 'FuncEntry's that belonged to different
functions.  Fortunately, the machine type checker caught this error.

The new main function now gets a FuncEntry, which updated in the
RSSA.Program.main recored.