fix(tvix/eval): handle thunks in arithmetic builtins

The simplest solution seems to be to pass references to arithmetic_op!()
which avoids the moving annoyance we had to deal with in the
builtins (no more popping!). We then use .force() to force the values
and dereference any Thunks (which arithmetic_op! doesn't do for us).

Change-Id: I0eb8ad60e80a0b3ba9d9f411e973ef8bcf136989
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6724
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.exp

@@ -1 +1 @@
-[ 18 18.9 18.9 19.1 19 ]
+[ 18 18.9 18.9 19.1 19 42 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.nix

@@ -4,4 +4,5 @@
   (builtins.add 7 11.9)
   (builtins.add 7.2 11.9)
   (builtins.add 7.1 11.9)
+  (builtins.add (builtins.add 21 10) 11)
 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.exp

@@ -1 +1 @@
-[ 3 7 0 1 0 0.5 0.5 0.5 ]
+[ 3 7 0 1 0 0.5 0.5 0.5 42 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.nix

@@ -7,4 +7,5 @@
   (builtins.div 1.0 2)
   (builtins.div 1 2.0)
   (builtins.div 1.0 2.0)
+  (builtins.div (builtins.div 84 4) 0.5)
 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.exp

@@ -1 +1 @@
-[ 36 0 0 14 ]
+[ 36 0 0 14 42 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.nix

@@ -3,4 +3,5 @@
   (builtins.mul 0 7)
   (builtins.mul 7 0)
   (builtins.mul 7 2)
+  (builtins.mul (builtins.mul 4 0.5) 21)
 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.exp

@@ -1 +1 @@
-[ -4 -3.1 -4.9 -4.7 -4 ]
+[ -4 -3.1 -4.9 -4.7 -4 42 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.nix

@@ -4,4 +4,5 @@
   (builtins.sub 7 11.9)
   (builtins.sub 7.2 11.9)
   (builtins.sub 7.9 11.9)
+  (builtins.sub (builtins.sub 123 23) 58)
 ]