High performance VM

[bobbimanners]

The reference implementation of the VM is not optimized for speed and has loads of paranoid checks.
Once things stabilize I will attempt an optimized version in 6502 assembly.

[bobbimanners]

I have started down this road:

1. using a jump table for VM instruction dispatch.
2. moving common registers and eval stack to zero page.
3. hand-optimized assembler to shave a few instructions
   This work is in v0.78.