fix(tvix/eval): place plain inherits in correct stack slots

We need to make sure that we compile all plain inherits in a let
expression before declaring any other locals. Plain inherits are special
in the sense that they can never be recursive, instead resolving to a
higher scope. Thus we need to compile their value, before declaring
them. If we don't do that, before any other local can be declared,
we cause a situation where the plain inherits' values are placed into
other locals' stack slots.

Note that we can't integrate the plain inherit compilation into the
regular 2-3 phase model where we defer the compilation of the value or
we'd compile `let inherit x; in …` as `let x = x; in …`.

Change-Id: I951d5df3c9661a054e12401546875f4685b5bf08
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6496
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.exp

@@ -0,0 +1 @@
+[ 1 2 3 4 ]

tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.nix

@@ -0,0 +1,20 @@
+# This test mixes different ways of creating bindings in a let … in expression
+# to make sure that the compiler initialises the locals in the same order as
+# they are declared.
+
+let
+  d = 4;
+in
+
+# Trick to allow useless inherits in the following let
+with { _unused = null; };
+
+let
+  set = { b = 2; };
+  a = 1;
+  inherit (set) b;
+  c = 3;
+  inherit d;
+in
+
+[ a b c d ]