Add no_std support so rc-zip can be used in embedded enviroments.

[jacobguenther]

Motivation:
Reading zip files on SD cards on micro controllers.

Work Required:
The only non optional dependency that actually needs the standard library is oval. I haven't looked at the optional dependencies yet. All of the other dependencies have a feature flag "std" enabled by default which can be disabled.

Oval is a very small library and could be re-implemented to not use the std library. It relies on std::io::{Read, Write, Result}. These could be replaced with embedded_io Read Write and Error. The core library does not have traits for Read and Write.

A new feature would need to be added, normally called "std". And enabled by default.

Something like
`
#[cfg(not(feature = "std"))]
extern crate alloc;
#[cfg(not(feature = "std"))]
use alloc::borrow::Cow;
#[cfg(feature = "std")]
use std::borrow::Cow;

#[cfg(not(feature = "std"))]
use core::cmp;
#[cfg(feature = "std")]
use std::cmp;
`
would have to be done. Or just use the core library.