feat(tvix/eval): compile with expression

Adds an additional structure to the compiler's scope to track the
runtime "with stack", i.e. the stack of values through which
identifiers should be dynamically resolved within a with-scope.

When encountering a `with` expression, the value from which the
bindings should be resolved is pushed onto the stack and tracked by
the compiler in the "with stack", as well as with a "phantom value"
which indicates that the stack contains an additional slot which is
not available to users via identifiers.

Runtime handling of this is not yet implemented.

Change-Id: I5e96fb55b6378e8e2a59c20c8518caa6df83da1c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6217
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -52,6 +52,9 @@ pub enum OpCode {
     OpAttrOrNotFound,
     OpAttrsIsSet,
 
+    // `with`-handling
+    OpPushWith(usize),
+
     // Lists
     OpList(usize),
     OpConcat,