Consider using only 1 bit per argument in the Argument Encoding Bitmap

[popematt]

We could potentially make most macro invocations even smaller in binary Ion by using only one bit per argument in the Argument Encoding Bitmap (AEB), but the tradeoff is that a void argument for a * parameter would likely be larger, and it could introduce more complexity into (and slow down) the binary reader and writer.
The interpretation of the AEB could be something like the following:

Cardinality	0	1
!	n/a	n/a
?	void	single expression
+	expression group	single expression
*	expression group	single expression