Handle ObjectBegin/ObjectEnd and ObjectInstance in parser

[wahn]

The vw-van.pbrt test scene contains those keywords and we have to handle them in the parser:

$ rg ObjectBegin | head -n 1
vw-van.pbrt:    ObjectBegin "Null.60"
$ rg ObjectEnd | head -n 1
vw-van.pbrt:ObjectEnd
$ rg ObjectInstance | head -n 1
vw-van.pbrt:                    ObjectInstance "Null.60"

[wahn]

Commit ec012b3 provides a minimal implementation.

Let's close the issue until we find a scene which really uses object instances. The VW scene from issue #56 does not seem to use the instances, at least I can't see any difference between the C++ and Rust version.