feat(tvix/eval): implement DisassemblingObserver for compiler

This type implements an observer that is called whenever the compiler
emits a chunk (after the toplevel, thunks, or lambdas) and prints the
output of the disassembler to its internal writer.

This replaces half of the uses of the `disassembler` feature, which
has been removed from the Cargo configuration.

Note that at this commit runtime tracing is not yet implemented as an
observer.

Change-Id: I7894ca1ba445761aba4ad51d98e4a7b6445f1aea
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6449
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/lib.rs

@@ -1,6 +1,7 @@
 mod builtins;
 mod chunk;
 mod compiler;
+mod disassembler;
 mod errors;
 mod eval;
 mod observer;
@@ -10,9 +11,6 @@ mod value;
 mod vm;
 mod warnings;
 
-#[cfg(feature = "disassembler")]
-mod disassembler;
-
 #[cfg(test)]
 mod tests;