Decide whether unused AEB bits must be zero

[tgregg]

The argument encoding bitmap requires (as of its current definition) 2 bits per non-required parameter. For macro signatures with numbers of non-required parameters not a multiple of 4, this leaves some unused bits in the argument encoding bitmap byte(s).

Should implementation(s) require these unused bits to be zero, or simply ignore them? Currently ion-java ignores them. The spec should probably take a stance here as well.

The following data illustrates this:

  0x01, // Macro invocation 1 ($ion::values, when system macros are in scope)
  0xF0  // The argument encoding bitmap, with the lowest 2 bits (`00`) required by the single zero-to-many parameter

If unused bits are ignored, this is valid and expands to nothing (i.e., (:values)). If unused bits must be zero, then this must raise an error.