S16x4a

After more than a year, the S16X4 finally gets some of its promised upgrades.  Included:

1) Subroutine call instructions: LCALL for calling +/- 4KB from current PC, and ICALL for calling anywhere.
2) ABORT_I hardware input to abort current instruction and packet, forcing instruction fetch in next cycle.  Used by MMU and/or debug exception hardware.
3) Parameter stack depth increased to six, to facilitate interrupt handlers with zero-cost context switching.