size

Bump to version 0.5.0-preview1

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.

Add missing word from README.md

This is a major (and slightly breaking) update to the PrettySize size crate,
bringing significant new features, improvements to the API, and a complete
overhaul of the documentation.

The major changes from v0.1.x are outlined below:

• Support for mathematical operations on strongly-typed Size<T> sizes, that
  include support for adding and subtracting Size instances and for
  multiplying/dividing Size instances by scalar values,
• Support (both for creating/handling and for formatting/printing) negative file
  sizes, required to express the potentially negative difference between two
  Size values,
• Size now implements PartialEq so Size values can be compared directly
  (instead of having to compare size1.bytes() and size2.bytes()),
• Abbrevations are now available for the various strongly-typed Size
  discriminants, e.g. you can use let foo = Size::KiB(2); instead of let foo = Size::Kibibytes(2) (the resulting type is still the same for both),
• The scalar constants have been moved from the crate root to size::consts to
  discourage their use in favor of the strongly-typed Size<T> approach,
• Size instances can be compared for equality both directly and via
  references,
• Size instances can now be compared for order (via PartialOrd),
• The size crate can now be used in no_std mode by compiling with default
  features disabled (see the crate-level documentation for more info),
• All types and methods have been extensively documented in addition to
  extensive crate- and module-level documentation.

The definition for TERABYTE was incorrect thanks to a fat-fingered
copy-and-paste, leading to future base-10 definitions being off by a
factor of 10^3. Thanks to @teschty for catching this.

Switch to u64 for all constants