doc/INTERNALS.md rewording for volatile functions
gettuple: use ASM_TPL_IMM to encode int<1024 and use >1024 for reg indices

so we have 0-1023 for direct indices and 1024-2055 for regs.
add better opcode descriptions in vm.c
gettuple is now only for fast unchecked PNTuple access, no bounds checks
no negative indices.
Improve potion_x86_debug: add rcx, rdx.
vm: mark reg+1 as reserved when gettuple needs reg+1.
jit still broken for the prev. getlocal (wrong %rax)