refactor(tvix/eval): add SourceCode directly into error types

With this change it's no longer necessary to track the SourceCode
struct separately from the evaluation for error reporting: It's just
stored directly in the errors.

This also ends up resolving an issue in compiler::bindings, where we
cloned the Arc containing file references way too often. In fact those
clones probably compensate for all additional SourceCode clones during
error construction now.

Change-Id: Ice93bf161e61f8ea3d48103435e20c53e6aa8c3a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10986
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -293,10 +293,11 @@ pub struct Error {
     pub kind: ErrorKind,
     pub span: Span,
     pub contexts: Vec<String>,
+    pub source: SourceCode,
 }
 
 impl Error {
-    pub fn new(mut kind: ErrorKind, span: Span) -> Self {
+    pub fn new(mut kind: ErrorKind, span: Span, source: SourceCode) -> Self {
         let mut contexts = vec![];
         while let ErrorKind::WithContext {
             context,
@@ -311,6 +312,7 @@ impl Error {
             kind,
             span,
             contexts,
+            source,
         }
     }
 }