Add '_allow' record/class field, which turns off field access checks

Also change ad-hoc condition to turn them off for class structs to use
this field instead.
Error out when trying to marshal nil to non-optional callback

Previously we created C closure which attempted to call Lua nil and
threw, which is not very useful behaviour.
Add support for custom callback definition

Similar to already existing custom C function description, but used
for callbacks from C to Lua.
Add FFI type for GStrv
Fix SoupSvr sample, it was still using wrong read_async signature
Add support for GLib.Markup parsing

Requires quite a lot of lgi internals trickery because GLib.Markup is
ancient and quite binding-unfriendly API.