This trait will enable library users of tvix-eval to observe internal
happenings of the compilation and runtime processes.
The initial methods of the observer will be called whenever the
compiler emits a chunk.
Change-Id: I668f6c2cfe3d6f4c1a1612c0f293831011768437
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6448
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
