Low level parsing crate

[Plecra]

I've been considering moving the core-friendly parsing logic into a dependency crate, which could be used directly when the main crate can't support a use case directly. It wouldn't be stabilized with the main crate.

// brainstorming
use zip_format::*;

let mut archive = std::fs::File::open("archive.zip").unwrap();
let footer16_offset = locate_footer16(&mut archive)
    .ok_or(Error::not_an_archive())?;
let (footer16, comment_length) = Footer16::parse(&mut archive)?;
footer16.validate()?; // Multi-disk support is not supported

let comment = {
    let mut buf = Vec::with_capacity(comment_length);
    reader.take(comment_length.into()).read_to_end(&mut buf)?;
    buf
};
let footer = Footer64Locator::parse(&mut archive, footer16_offset)?
    // validation that single disk & locator.footer_offset < offset_of(locator)  
    .filter(|d| d.validate(&footer16).is_ok())
    // TODO: use extensible data sector.
    .map(|loc| Footer64::parse(loc.footer_offset))
    .transpose()?
    // Ignore footer64 if fields <FFFF dont match
    .map(|footer64| Footer::combine(&footer16, footer64))
    .unwrap_or_else(|| Footer::from(&footer));