treewide: refactor read_file and mkdir

[KarlK90]

mkdir and read_file augment the error case with the path in the error case. This is useful as we otherwise don't know which location failed. This commit refactors the functions to take any argument that coerces into a Path - which is implemented by many types.

mkdir now uses create_dir_all as it is not meant to fail if the directory already exists.

builds/depends on #5

[KarlK90]

For nicer error messages I propose to add https://crates.io/crates/anyhow to the project which is the canonical error handling library for Rust applications. It adds a bit to the final binary size but allows attaching context objects/information to the error cases and has better ergonomics for producing errors e.g.:

pub(crate) fn read_file(filename: impl AsRef<Path>) -> Result<String> {
    read_to_string(filename.as_ref())
        .with_context(|| format!("Failed to read {}", filename.as_ref().to_string_lossy()))
}

Gives in this simple error case (which can be extended easily):

Error: Failed to read test_test

Caused by:
    Is a directory (os error 21)

instead of:

Error: "Failed to read test_test: Is a directory (os error 21)"

[michaelolbrich]

For nicer error messages I propose to add https://crates.io/crates/anyhow to the project which is the canonical error handling library for Rust applications. It adds a bit to the final binary size but allows attaching context objects/information to the error cases and has better ergonomics for producing errors e.g.:

pub(crate) fn read_file(filename: impl AsRef<Path>) -> Result<String> {
    read_to_string(filename.as_ref())
        .with_context(|| format!("Failed to read {}", filename.as_ref().to_string_lossy()))
}

Gives in this simple error case (which can be extended easily):

Error: Failed to read test_test

Caused by:
    Is a directory (os error 21)

instead of:

Error: "Failed to read test_test: Is a directory (os error 21)"

I want to keep the binary size down and adding extra stuff for nicer error messages does not sound like a good idea.

[michaelolbrich]

So You're basically doing 3 things in this commit:

1. move thing into a separate file
2. use create_dir_all
3. Change the argument to AsRef<Path>

I've done some size measurements with this. I've used armv7-unknown-linux-musleabihf as target for this.

The size of the text section grows by 2.5k and the file size by 4k (I think there is some 4k alignment in there).
That's a bit much for be.

A big chunk is create_dir_all because it does more than we actually need. But the AsRef stuff causes a lot as well and it's not quite clear to me why.

So for now, please just factor out the stuff into a separate file and lets look at the other cleanup in a different PR and see if we can come up with something that costs less.