Remove bitflags dependency

[npmccallum]

The only real use of bitflags was to save a tiny bit of memory. It isn't
exposed externally and we can just use a flag enumeration combined with
a HashSet to replace it. Because of this, let's not subject this
dependency to all projects which depend on clap.

[npmccallum]

This PR is an adaptation of #1436 to the v3 branch.

[npmccallum]

@spacekookie The test failures are unrelated.

[kbknapp]

@npmccallum I'm going to close this as per the discussion in #1436 however I'm 100% open to a local bitflags like solution against the v3-master branch.

All we really need out of a bit flags style solutions is:

• the ability to map an enum to one or more bits of the field, bonus if the width can vary using macros. However it's fine we just pick an arbitrary width like 64bit
• the ability to set/unset bits
• the ability to check if a particular bit(s) is set or not

[BurntSushi]

@kbknapp I did something super simple with a tiny macro for regex-syntax: https://github.com/rust-lang/regex/blob/9687986de45378c744180cc696286aad7db83c5e/regex-syntax/src/hir/mod.rs#L1425-L1473 --- Not sure if it works for clap, though, it might!