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/opcode.rs

@@ -72,4 +72,7 @@ pub enum OpCode {
 
     // Close scopes while leaving their expression value around.
     OpCloseScope(usize), // number of locals to pop
+
+    // Asserts stack top is a boolean, and true.
+    OpAssert,
 }