feat(tvix/eval): implement (compilation) observer trait

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

tvix/eval/src/lib.rs

@@ -3,6 +3,7 @@ mod chunk;
 mod compiler;
 mod errors;
 mod eval;
+mod observer;
 mod opcode;
 mod upvalues;
 mod value;