refactor(corp/tvixbolt): adapt for tvix-eval's upcoming observer API

Instead of the previous hack which painfully threaded through
a structure that the disassembler could write to, Tvix's evaluator is
gaining a new "Observer" API which lets library clients observe
compilation output (and, soon!, runtime tracing).

This adapts tvixbolt to use this observer interface (with the default
`DisassemblingObserver`) to populate the `bytecode` field of its
output.

This is purely a mechanical change, no functionality is impacted.

Change-Id: I22bd2218629f30fd7351d4cc5ddcf639c12fea14
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6316
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

corp/tvixbolt/Cargo.toml

@@ -17,4 +17,3 @@ rev = "97b438e34be5211a4b48aeed9cc3ded489b4d6da"
 [dependencies.tvix-eval]
 path = "../tvix-eval/tvix/eval"
 default-features = false
-features = [ "disassembler" ]