feat(tvix/eval): introduce mechanism for defining builtins

Adds a new builtins module in which builtins can be constructed. The
functions in this module should return a correctly structured value to
be passed to the compiler's `globals`.

This is wired up all the way to the compiler with an example
`toString` builtin, available as a global. Note that this does not yet
actually behave like the real toString, which has some differences
from `Display`.

Change-Id: Ibb5f6fbe6207782fdf2434435567fc1bd80039a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6254
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/builtin.rs

@@ -34,6 +34,19 @@ pub struct Builtin {
 }
 
 impl Builtin {
+    pub fn new(name: &'static str, arity: usize, func: BuiltinFn) -> Self {
+        Builtin {
+            name,
+            arity,
+            func,
+            partials: vec![],
+        }
+    }
+
+    pub fn name(&self) -> &'static str {
+        self.name
+    }
+
     /// Apply an additional argument to the builtin, which will either
     /// lead to execution of the function or to returning a partial
     /// builtin.