gpio-cdev

• Removed pub "errors" module. Error now exposed at top level.
• MSRV is now 1.39.0
• Add support behind a feature flag for reading events from a line as a Stream via tokio. #35.

Refactored Errors:

• Removed the error-chain dependency.
• Errors are now implemented "manually" with ErrorKind and IoctlKind enums.
• The encompassing Error type implements the std::error::Error trait.

Adds the ability to create a collection of lines from a single chip and read or write those lines simultaneously with a single stystem call.

• A new Lines object (plural) was added. It is a collection of individual Line objects on a single Chip which can be read or written simultaneously with a single system call.
• A Line now just contains the reference to the Chip and the offset number. No system call is incurred when one is created.
• Information about an individual line is now represented by a separate LineInfo struct which can be obtained from the function Line::info(). This incurs a system call to retrieve the information.
• Creating a Line can't fail unless the caller specifies an offset that is out of range of the chip.
• The LineIterator can not fail since it checks the offset range. So now its item is just a Line, and not Result<Line>.
• There was no longer a need for Line::refresh() so it was removed.
• Since a Line object is trivial to create, it is now OK to have Lines be a simple collection of Line structs.

• Initial release of the library with basic operations centered around operating on a single line at a time.