feat(tvix/eval): implement compilation of upvalue access

This adds a new upvalue tracking structure in the compiler to resolve
upvalues and track their positions within a function when compiling a
closure.

The compiler will emit runtime upvalue access instructions after this
commit, but the creation of the runtime closure object etc. is not yet
wired up.

Change-Id: Ib0c2c25f686bfd45f797c528753068858e3a770d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6289
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/function.rs

@@ -7,6 +7,7 @@ use crate::chunk::Chunk;
 pub struct Lambda {
     // name: Option<NixString>,
     pub(crate) chunk: Rc<Chunk>,
+    pub(crate) upvalue_count: usize,
 }
 
 impl Lambda {
@@ -14,6 +15,7 @@ impl Lambda {
         Lambda {
             // name: None,
             chunk: Default::default(),
+            upvalue_count: 0,
         }
     }