feat(tvix/eval): implement assert operator

This implements `assert`, which evaluates an expression and aborts
evaluation if the value is not `true`.

At this point we should introduce eval-failed-* tests; probably
asserting against some representation of the error enum?

Change-Id: If54c8f616d89b829c1860a4835dde60a2cd70d7a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6230
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -33,6 +33,8 @@ pub enum Error {
     UnknownDynamicVariable(String),
 
     ParseErrors(Vec<rnix::parser::ParseError>),
+
+    AssertionFailed,
 }
 
 impl Display for Error {