feat(tvix/eval): introduce initial EvalIO trait

This trait is going to be used to abstract filesystem interactions in
Tvix.

For now, it only contains a `read_to_string` method that closely
mirrors `std::fs::read_to_string`.

As a first step, to see how this works in practice, we will thread
through only this function to the various relevant parts.

Two implementations are provided in tvix-eval itself: A dummy
implementation (which just returns ErrorKind::NotImplemented for all
operations), and a std implementation which delegates to `std`
functions.

Change-Id: Ied3e3bf4bd0e874dd84e166190e3873a0f923ddb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7565
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/lib.rs

@@ -16,6 +16,7 @@ mod builtins;
 mod chunk;
 mod compiler;
 mod errors;
+mod io;
 pub mod observer;
 mod opcode;
 mod pretty_ast;