feat(tvix/eval): Record formals on lambda

In preparation for both implementing the `functionArgs` builtin and
adding support for validating closed formals, record information about
the formal arguments to a function *on the Lambda itself*. This may seem
a little odd for the purposes of just closed formal checking, but is
something we have to have anyway for builtins.functionArgs so I figured
I'd do it this way to kill both birds with one stone.

Change-Id: Ie3770a607bf352a1eb395c79ca29bb25d5978cd8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7001
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -20,6 +20,7 @@ use crate::opcode::StackIdx;
 use crate::vm::VM;
 pub use attrs::NixAttrs;
 pub use builtin::Builtin;
+pub(crate) use function::Formals;
 pub use function::{Closure, Lambda};
 pub use list::NixList;
 pub use path::canon_path;