Version v2.0.1: fix rust 2018 idioms

Version v2.0.0: new Rust conventions

1. `cause` is removed, use `source`
2. No `description()` any more
3. Compiler warnings of edition 2018 eliminated

Version v1.2.3: fixes warnings of recent rustc

This includes:
* ignore `description()` in macro (don't implement method)
* silence a warning on `cause()` method

Version v1.2.2: fixes lints for new rust

Version v1.2.1: Added `allow(unused_doc_comments)` for newer rust

Version v1.2.0: adding `wraps` constructor (private error types) and …

…supports conditional compilation

Version v1.1.0: added contexts

Version v1.0.0: Just a declaration that API is stable

Version v0.2.2: Small bugfix release

* Fix bug in new rust nightly
* Allow trailing comma in structs
* Relicensed code to MIT/Apache-2.0 instead of just MIT

Version v0.2.1: a trailing comma in struct variants and other minor f…

…ixes