The `Error` derive can be made to work well for the most part in
`no_std` environments by enabling `#![feature(error_in_core)]`. This
changes the `Error` derive slightly to import `Error` and related
traits from core, when the `std` feature is disabled.

Fixes #261

This adds the following line to all of our test files.
```rust
 #![cfg_attr(not(feature = "std"), no_std)]
```

This way we test that all our features also work in no_std environments.