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/Cargo.toml

@@ -17,7 +17,7 @@ smol_str = "0.1"
 rustyline = { version = "10.0.0", optional = true }
 dirs = "4.0.0"
 path-clean = "0.1"
-tabwriter = { version = "1.2", optional = true }
+tabwriter = "1.2"
 rowan = "*" # pinned by rnix
 codemap = "0.1.3"
 
@@ -43,9 +43,6 @@ nix_tests = []
 # Enables building the binary (tvix-eval REPL)
 repl = [ "dep:rustyline" ]
 
-# Enables printing compiled code and tracing the stack state at runtime.
-disassembler = ["dep:tabwriter"]
-
 [[bench]]
 name = "eval"
 harness = false