Simplify reading file contents to a String

[mjgarton]

This uses a simpler approach to reading the file contents to a String, making the code slightly more readable by removing code unrelated to the point of the example.

[sdroege]

I would've just use std::fs and then fs::read_to_string() to make it clearer where the function comes from but either way is fine.

This was indeed unnecessarily convoluted :)

[bilelmoussaoui]

let us land this as is, feel free to submit future improvements as separate PRs