feat(tvix/eval): Give names to builtin arguments

Refactor the arguments of a Builtin to be a vec of a new BuiltinArgument
struct, which contains the old strictness boolean and also a static
`name` str - this is automatically determined via the ident for the
corresponding function argument in the proc-macro case, and passed in in
the cases where we're still manually calling Builtin::new.

Currently this name is unused, but in the future this can be used as
part of a documentation system for builtins.

Change-Id: Ib9dadb15b69bf8c9ea1983a4f4f197294a2394a6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7204
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/impure.rs

@@ -12,7 +12,7 @@ use crate::{
     compiler::GlobalsMap,
     errors::ErrorKind,
     observer::NoOpObserver,
-    value::{Builtin, NixAttrs, Thunk},
+    value::{Builtin, BuiltinArgument, NixAttrs, Thunk},
     vm::VM,
     SourceCode, Value,
 };
@@ -111,7 +111,10 @@ pub fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builti
 
     Builtin::new(
         "import",
-        &[true],
+        &[BuiltinArgument {
+            strict: true,
+            name: "path",
+        }],
         move |mut args: Vec<Value>, vm: &mut VM| {
             let mut path = super::coerce_value_to_path(&args.pop().unwrap(), vm)?;
             if path.is_dir() {