Provide replacement for GAP kernel functions dealing with "iostream" (for launching and communicating with child processes)

[fingolfin]

Similar to issue #240, we should provide alternatives to GAP's iostream functions, which also are about launching and interacting with child processes.

One way to go about this would be to replace all the kernel functions provided by that module:

    GVAR_FUNC(CREATE_PTY_IOSTREAM, 3, "dir, prog, args"),
    GVAR_FUNC(WRITE_IOSTREAM, 3, "stream, string, len"),
    GVAR_FUNC(READ_IOSTREAM, 2, "stream, len"),
    GVAR_FUNC(READ_IOSTREAM_NOWAIT, 2, "stream, len"),
    GVAR_FUNC(KILL_CHILD_IOSTREAM, 1, "stream"),
    GVAR_FUNC(CLOSE_PTY_IOSTREAM, 1, "stream"),
    GVAR_FUNC(SIGNAL_CHILD_IOSTREAM, 2, "stream, signal"),
    GVAR_FUNC(IS_BLOCKED_IOSTREAM, 1, "stream"),
    GVAR_FUNC(FD_OF_IOSTREAM, 1, "stream"),

Alternatively, we could replace the GAP-level code wrapping these low-level functions; it is completely contained in GAP's lib/streams.gi, see function InputOutputLocalProcess and everything dealing with IsInputOutputStreamByPtyRep.