decode returns Result instead of Option (#52)

* decode returns Result instead of Option

Fixes #33

* don't use String inside Error

* some fixes to formatting and error messages

* fix review

* implements std::error::Error and std::fmt::Display for Error
* removes implementation of ToString
* fixes error message for decoding Enum
* generate error messages with format!() outside of the quote!()

* refactors error message generation

* adds simple sanity tests

* adds from str implementation for error

* adds tests for named structs fields

* remove all the literal error messages from no_std builds

* brings back PartialEq lost along the way

* fix review v3