refactor(tvix/eval): clone the Arc<codemap::File> for the compiler

This disconnects ownership of the `File` reference in a compiler from
the calling scope, which is required for when we implement `import`.

`import` will need to carry an `Rc<RefCell<CodeMap>>` (or maybe, in
the future, Arc) to give us the ability to add new detected code
files at runtime.

Note that the choice of `Arc` over `Rc` here is not ours - it's the
codemap crate's.

Change-Id: I3aeca4ffc167acbd1701846a332d93550b56ba7d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6630
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler/spans.rs

@@ -77,8 +77,8 @@ expr_to_span!(ast::Str);
 expr_to_span!(ast::UnaryOp);
 expr_to_span!(ast::With);
 
-impl Compiler<'_, '_> {
+impl Compiler<'_> {
     pub(super) fn span_for<S: ToSpan>(&self, to_span: &S) -> Span {
-        to_span.span_for(self.file)
+        to_span.span_for(&self.file)
     }
 }