core: add fs_err dependency

[tony-iqlusion]

The fs_err crate is a drop-in replacement for std:::fs with zero transitive dependencies which provides error messages that include the filename in addition to the failing operation:

https://github.com/andrewhickman/fs-err

This commit re-exports it as abscissa_core::fs and uses it to implement all framework-level filesystem operations.

Example

let file = File::open("does not exist.txt")?;

std::fs::File error message

The system cannot find the file specified. (os error 2)

fs_err::File error message

failed to open file `does not exist.txt`
    caused by: The system cannot find the file specified. (os error 2)

cc @yaahc

[tony-iqlusion]

Additional questions: should it be added to the prelude?

[yaahc]

@tony-iqlusion I think in theory adding it to the prelude could be a breaking change though its hard to imagine anyone would write code that triggers the issue. In theory tho i think this might fail to compile:

use std::*;
use abscissa::prelude::*; // error ambiguous `fs`

[tony-iqlusion]

Yeah, good point. I plan on starting to make breaking changes soon, but it might be good to make one more maintenance release before that.