feat(tvix/vm): add opcodes for new Value::NotFound sentinel

This sentinel value is going to be used for certain nested accesses
into attribute sets.

There is a new instruction similar to `OpAttrsSelect` which leaves the
sentinel on the stack if a key is not found, instead of raising an
error.

Additionally, a new jump instruction makes its jump operation
conditional on finding such a sentinel value.

Change-Id: I2642f0a0bcc85bbe0ead68ea09a7dd794dbedeac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6166
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -41,12 +41,14 @@ pub enum OpCode {
     OpJump(usize),
     OpJumpIfTrue(usize),
     OpJumpIfFalse(usize),
+    OpJumpIfNotFound(usize),
 
     // Attribute sets
     OpAttrs(usize),
     OpAttrPath(usize),
     OpAttrsUpdate,
     OpAttrsSelect,
+    OpAttrOrNotFound,
     OpAttrsIsSet,
 
     // Lists