feat(tvix/eval): implement Value::coerce_to_path()

This function is necessary for all builtins that expect some form of
path as an argument. It is merely a wrapper around coerce_to_string that
can shortcut if we already have a path. The absolute path check is done
in the same way as in C++ Nix for compatibility, although it should
probably be revised in the long term (think about Windows, for example).

Since coercing to a path is not an operation possible in the language
directly, this function can live in the builtins module as the only
place it is required.

Change-Id: I69ed5455c00d193fea88b8fa83e28907a761cab5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6574
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/string.rs

@@ -2,7 +2,7 @@
 //! backing implementations.
 use smol_str::SmolStr;
 use std::hash::Hash;
-use std::{borrow::Cow, fmt::Display};
+use std::{borrow::Cow, fmt::Display, str::Chars};
 
 #[derive(Clone, Debug)]
 enum StringRepr {
@@ -97,6 +97,13 @@ impl NixString {
         s.push_str(other.as_str());
         NixString(StringRepr::Heap(s))
     }
+
+    pub fn chars(&self) -> Chars<'_> {
+        match &self.0 {
+            StringRepr::Heap(h) => h.chars(),
+            StringRepr::Smol(s) => s.chars(),
+        }
+    }
 }
 
 fn nix_escape_char(ch: char, next: Option<&char>) -> Option<&'static str> {