Add mslice.parseHSlice, cligen/sysUt.cfor, $MSlice(nil, 0) fix
Besides those three headliner items (also detailed in RELEASE_NOTES.md and with full details in the git log), also make some libs work with Nim-1.2 era client code again.
Besides those three headliner items (also detailed in RELEASE_NOTES.md and with full details in the git log), also make some libs work with Nim-1.2 era client code again.