prettysize-rs 0.4.1

This update includes some hidden improvements as well as a few public-facing API changes.

New features:

• Size now implements AddAssign, SubAssign, MulAssign, and DivAssign.
• SizeFormatter::new() is now a const function.

Improvements:

• Crate documentation has been improved further.
• The MSRV (which is not guaranteed) has been reduced and currently the crate
  builds with rust 1.46.0.
• Crate tests have been modified to pass on older versions of the rust compiler.
• The FormattableSize and SizeFormatter APIs have been coalesced into the
  latter, reducing duplication and simplifying crate internals.

Special thanks to first-time contributors @angusholder and @vic1707.