feat(tvix/eval): implement initial compiler::optimiser module

This optimiser can rewrite some expressions into more efficient forms,
and warn users about those cases.

As a proof-of-concept, only some simple boolean comparisons are
supported for now.

Change-Id: I7df561118cfbad281fc99523e859bc66e7a1adcb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7766
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.exp

@@ -0,0 +1 @@
+[ true true false false true true false false ]

tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.nix

@@ -0,0 +1,21 @@
+let
+  makeTrue = _: true;
+  makeFalse = _: false;
+in
+[
+  # useless `false`
+  (false || makeTrue null) # true
+  (makeTrue null || false) # true
+
+  # useless `true`
+  (true && makeFalse null) # false
+  (makeFalse null && true) # false
+
+  # useless `||`
+  (true || makeFalse null) # true
+  (makeFalse null || true) # true
+
+  # useless `&&`
+  (false && makeTrue null) # false
+  (makeTrue null && false) # false
+]