Allow customizing file loading using the new Context trait

[alvra]

This change allows customizing how files are loaded.

Changes:

• Add a new trait Context, implemented by FileContext.
• Change the public signature of the method Format.write_root() (and related private ones) to accepting this trait instead of a FileContext directly.
• Add a new function parse_scss_readable(), like do_parse_scss_file() but accepting a Read.
  I was considering renaming the original functions from X_file() to X_path() since they accepts paths instead of files. Then the new ones that accept Read can become X_file().
  This would be a breaking change, however.
• Add a function parse_imported_scss_readable(), like parse_imported_scss_file()
  This function isn't used or public so it causes a warning, it can probably be removed.
• Change a field of Error::Input, from a PathBuf to a String.
  This should be the only backwards incompatible change.

Example of how to use this

given:

use std::collections::HashMap;
use rsass::{Context, Error};

#[derive(Clone, Debug)]
struct StaticContext<'a> {
    files: HashMap<String, &'a[u8]>,
}

impl<'a> Context for StaticContext<'a> {
    type File = &'a [u8];

    fn find_file(
        &self, name: &str
    ) -> Result<Option<(Self, String, Self::File)>, Error> {
        if let Some(file) = self.files.get(name).map(|data| *data) {
            Ok(Some((self.clone(), name.to_string(), file)))
        } else {
            Ok(None)
        }
    }
}

then:

let format = ...
let entry = "import 'a'; import 'b';".as_bytes();
let file_a = "a { color: red }".as_bytes();
let file_b = "p { color: blue }".as_bytes();
let mut context = StaticContext { files: HashMap::new() };
context.files.insert("a".to_string(), file_a);
context.files.insert("b".to_string(), file_b);

output = format.write_root(
    parse_scss_data()?,
    &mut rsass::GlobalScope::new(self.format),
    context)?;
assert_eq!(
    output,
    format!("{}\n{}",
        std::str::from_utf8(file_a).unwrap(),
        std::str::from_utf8(file_b).unwrap()));

[kaj]

Thank you for your contribution! This is something I have considered for some time but not yet implemented, so thanks for that.

[kaj]

I think this is close to being merged now, just humor me with a few more nitpicks ...

[alvra]

I think I covered everything, to me it looks ready now.
Let me know if you need anything else :)

[kaj]

Yes, I think this looks good now. Thank you for the contribution!