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/vm.rs

@@ -175,7 +175,7 @@ impl<'o> VM<'o> {
 
     /// Construct an error from the given ErrorKind and the source
     /// span of the current instruction.
-    fn error(&self, kind: ErrorKind) -> Error {
+    pub fn error(&self, kind: ErrorKind) -> Error {
         Error {
             kind,
             span: self.current_span(),