refactor(tvix/eval): add VM::call_value helper method

This makes it possible to call a callable value (builtin or
closure/lambda) directly, without unwrapping it first. This is needed
for pretty much all higher-order functions to work correctly.

This is mostly equivalent to the previous code in coerce_to_string for
calling `__toString`, except it expects the argument(s) to already be
placed on the stack.

Note that the span for the `NotCallable` error is not currently
guaranteed to make any sense, will experiment with this.

Change-Id: I821224368d438a28900858b343defc1817e46a0a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6717
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

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

@@ -1 +1 @@
-[ [ 1 2 3 4 5 ] [ 2 4 6 8 10 ] [ 2 4 6 8 10 ] [ 1 2 3 4 5 ] ]
+[ [ 1 2 3 4 5 ] [ 2 4 6 8 10 ] [ 2 4 6 8 10 ] [ 2 4 6 8 10 ] [ 1 2 3 4 5 ] ]

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

@@ -11,6 +11,9 @@
     in builtins.map (x: x * n) [ 1 2 3 4 5 ]
   )
 
+  # same, but with a builtin
+  (builtins.map (builtins.mul 2) [ 1 2 3 4 5 ])
+
   # from global scope
   (map (x: x) [ 1 2 3 4 5 ])
 ]