feat(tvix/eval): implement 'builtins.filter'

This is a little ugly because the plain Iterator::filter method can
not be used (it does not support fallible primitives), so we need to
resort to an `Iterator::filter_map` and deal with the wrapping in
Options everywhere.

This prevents use of `?` which introduces the need for some matching,
but it's not *too* bad.

Change-Id: Ie2c3c0c9756c4c627176f64fb4e0054e717c26d1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6765
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

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

@@ -0,0 +1 @@
+[ [ 1 2 3 4 5 ] [ ] [ 2 2 2 ] [ [ 1 2 ] [ 3 4 ] ] ]

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

@@ -0,0 +1,13 @@
+[
+  (builtins.filter (_: true) [ 1 2 3 4 5 ])
+  (builtins.filter (_: false) [ 1 2 3 4 5 ])
+  (builtins.filter (x: x == 2) [ 1 2 1 2 1 2 ])
+
+  (builtins.filter (x: (builtins.length x) > 0) [
+    [ ]
+    [ 1 2 ]
+    [ ]
+    [ ]
+    [ 3 4 ]
+  ])
+]