feat(tvix/eval): implement attribute set access operator

Fairly straightforward, handling the optimised representations
manually and otherwise delegating to BTreeMap.

Note that parsing of raw identifiers is not yet implemented.

Encountering an identifier node usually means that there is locals
access going on, so we need a special case for compiling a node in
such a way that an identifier's literal value ends up on the stack.

Change-Id: I13fbab7ac657b17ef3f4c5859fe737c321890c8a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6158
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -10,6 +10,10 @@ pub enum Error {
         given: &'static str,
     },
 
+    AttributeNotFound {
+        name: String,
+    },
+
     TypeError {
         expected: &'static str,
         actual: &'static str,