bitflags breaking changes

serde: enable no-std support by @nim65s in https://github.com/bitflags/bitflags/pull/296

Add a parser for flags formatted as bar-separated-values by @KodrAus in https://github.com/bitflags/bitflags/pull/297

@rusty-snake made their first contribution in https://github.com/bitflags/bitflags/pull/291

@fintelia made their first contribution in https://github.com/bitflags/bitflags/pull/294

@nim65s made their first contribution in https://github.com/bitflags/bitflags/pull/296

Use SPDX license format by @atouchet in https://github.com/bitflags/bitflags/pull/272

serde tests fail in CI by @arturoc in https://github.com/bitflags/bitflags/pull/277

Fix beta test output by @KodrAus in https://github.com/bitflags/bitflags/pull/279

Prepare for 2.0.0-rc.2 release by @KodrAus in https://github.com/bitflags/bitflags/pull/299

Iterator over all the enabled options by @arturoc in https://github.com/bitflags/bitflags/pull/278

from_bits_(truncate) fail with composite flags by @arturoc in https://github.com/bitflags/bitflags/pull/276

Add more platform coverage to CI by @KodrAus in https://github.com/bitflags/bitflags/pull/280

rework the way cfgs are handled by @KodrAus in https://github.com/bitflags/bitflags/pull/281

Split generated code into two types by @KodrAus in https://github.com/bitflags/bitflags/pull/282

expose bitflags iters using nameable types by @KodrAus in https://github.com/bitflags/bitflags/pull/286

Support creating flags from their names by @KodrAus in https://github.com/bitflags/bitflags/pull/287

Update README.md by @KodrAus in https://github.com/bitflags/bitflags/pull/288

Prepare for 2.0.0-rc.1 release by @KodrAus in https://github.com/bitflags/bitflags/pull/289

Add missing "if" to contains doc-comment in traits.rs by @rusty-snake in https://github.com/bitflags/bitflags/pull/291

Forbid unsafe_code by @fintelia in https://github.com/bitflags/bitflags/pull/294

serde: enable no-std support by @nim65s in https://github.com/bitflags/bitflags/pull/296

Add a parser for flags formatted as bar-separated-values by @KodrAus in https://github.com/bitflags/bitflags/pull/297

Use strip_prefix instead of starts_with + slice by @QuinnPainter in https://github.com/bitflags/bitflags/pull/301

Add example to the README.md file by @tiaanl in https://github.com/bitflags/bitflags/pull/270

Fix a typo and call out MSRV bump by @KodrAus in https://github.com/bitflags/bitflags/pull/259

BitFlags trait by @arturoc in https://github.com/bitflags/bitflags/pull/220

Add a hidden trait to discourage manual impls of BitFlags by @KodrAus in https://github.com/bitflags/bitflags/pull/261

Sanitize `Ok` by @konsumlamm in https://github.com/bitflags/bitflags/pull/266

Fix bug in `Debug` implementation by @konsumlamm in https://github.com/bitflags/bitflags/pull/268

Fix a typo in the generated documentation by @wackbyte in https://github.com/bitflags/bitflags/pull/271

Other improvements to unit tests and documentation ([#106] and [#115])

**[breaking change]** Minimum supported version is Rust **1.20**, due to usage of associated constants

**[breaking change]** Macro now generates [associated constants](https://doc.rust-lang.org/reference/items.html#associated-constants) ([#24])

After being broken in 0.9, the `#[deprecated]` attribute is now supported again ([#112])

Printing an empty flag value with the `Debug` trait now prints "(empty)" instead of nothing ([#85])

The `bitflags!` macro can now be used inside of a fn body, to define a type local to that function ([#74])

**[breaking change]** Use struct keyword instead of flags to define bitflag types ([#84])

**[breaking change]** Terminate const items with semicolons instead of commas ([#87])

Implement the `Hex`, `Octal`, and `Binary` formatting traits ([#86])