Doesn't build in no_std env but it seems like it is supposed to build there

I wanted to use this library in a no_std environment, but it doesn't compile because of the miniz_oxide crate. If I see it correctly in Cargo.toml and lib.rs, this crate is supposed to work in no_std-environments.

I'd like to propose the following changes:

1. clarify in README.md if this crate actually supports no_std and what one has to do for it
2. if this is a bug, please fix it and perhaps add some CI which covers this in the future

The build error for my x86_64 no_std-target is:

Compiling miniz_oxide v0.4.4
error[E0463]: can't find crate for `std`
  |
  = note: the `x86_64-unknown-hedron-5835702812158589728` target may not support the standard library
  = note: `std` is required by `miniz_oxide` because it does not declare `#![no_std]`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

For more information about this error, try `rustc --explain E0463`. 

Start with Cargo.toml and lib.rs to verify the crate's advertised no_std configuration and inspect how miniz_oxide is included. Reproduce the reported x86_64 no_std build error, then check README.md and the CI configuration for the documented support and coverage expected by the issue.