WIP: Add true streaming

[rolandlo]

Tests are still missing.

The script example/target.lua works with luajit and Lua 5.1, 5.2 and 5.3, but crashes with Lua 5.4 due to illegal machine instruction.

[jcupitt]

This is very cool!

The custom source / target classes would need the g_signal_connect() etc. mechanism to be wrapped and this is significantly harder because of callbacks, GC integration, and marshaling.

Signals are handy for other stuff --- pyvips exposes the progress signals, so you can have on_eval() etc. handlers which tick down during evaluation. They are useful for desktop or CLI programs but probably less so on servers heh.