* Implement syscalls for BYTE, SIGNEXTEND, SAR, SLT and SGT.
* Implement SDIV and SMOD; minor documentation and tidying.
* Implement EXP.
* Add sys_byte to the syscall jumptable.
* Test suite for signed syscalls.
* Handle `EXIT_KERNEL` "properly".
* Add gas charges; rename label.
* Uppercase all opcodes.
* Add test for BYTE; fix bug in BYTE.
* Calculate and charge gas for calling `EXP`.
* Fix gas calculation for `exponent = 0`.
* Address Jacqui's comments.
* Remove BYTE syscall as it will be implemented natively.
* Oops, forgot to remove this bit.
