new register allocator and calling convention

[Stephen Weeks]

> Why is it hard to return values from calls to the runtime system?  The gmp
> stuff does it.

If a MLton primitive returns a value, then its call will appear on the
right hand side of a C assignment statement.  It's hard to fit C
statements inside of expressions -- the comma operator only allows
assignments before a comma.   Also, I have one macro which is used for
all runtime system invocations now, and it would be hard to put this
on the rhs of a C assignment.

#define MLTON_invokeRuntime(call, frameSize, ret)		\
	do {							\
		stackTop += (frameSize);			\
		*(uint*)stackTop = (ret ## _index);		\
		MLTON_flushGC();				\
		call;						\
		MLTON_cacheGC();				\
		MLTON_return();					\
		ret:						\
		stackTop -= (frameSize);			\
	} while (0)