builtins do not support being suspended (while other processes do)

[lmorg]

Currently there is code to handle suspending processes and a jobs function (albeit it's currently a murex function) but support hasn't been extended out to murex builtins - yet.

It shouldn't take much to get built ins to that stage but it may end up being a bit of a kludge where processes aren't stopped via SIGSTSP but rather "paused" via a channel (possible using context.Context?).