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/vm.rs

@@ -304,6 +304,12 @@ impl VM {
 
                     return Err(Error::UnknownDynamicVariable(ident.to_string()));
                 }
+
+                OpCode::OpAssert => {
+                    if !self.pop().as_bool()? {
+                        return Err(Error::AssertionFailed);
+                    }
+                }
             }
 
             #[cfg(feature = "disassembler")]