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
