refactor(tvix/eval): use EvalIO::read_to_string in impure builtins

With this change, the behaviour of reading a string from a file path
is controlled by the provided `EvalIO` structure.

This is a huge step towards abstracting away I/O behaviour correctly.

Change-Id: Ifde8e46cd863b16e0301dca45a434ad27560399f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7567
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/vm.rs

@@ -220,6 +220,11 @@ impl<'o> VM<'o> {
         self.chunk().get_span(self.frame().ip - 1)
     }
 
+    /// Access the I/O handle used for filesystem access in this VM.
+    pub(crate) fn io(&self) -> &Box<dyn EvalIO> {
+        &self.io_handle
+    }
+
     /// Returns the information needed to calculate the current span,
     /// but without performing that calculation.
     fn current_light_span(&self) -> LightSpan {