Add a parser for flags formatted as bar-separated-values

[KodrAus]

The last blocker to #262

This PR adds a parser for flags formatted using the default Debug impl, like:

A | B | C | 0x0e

It also updates the default serde implementations to use the text format and parser for human-readable formats.

I'll update this PR with more details once I've got the parser properly written and tested.

[KodrAus]

One reasonable enhancement we could consider in the future is introducing a trait for parsing:

pub trait Parser {
    type Bits;

    fn name(&mut self, flag: &str) -> Result<Self::Bits, ParseError>;
    fn hex(&mut self, flag: &str) -> Result<Self::Bits, ParseError>;

    fn whitespace(&mut self, ws: &str) -> Result<(), ParseError> {
        Ok(())
    }

    fn separator(&mut self, separator: &str) -> Result<(), ParseError> {
        Ok(())
    }
}

We could then add a method to our generated internal type (not the public one) like:

pub fn parse(parser: &mut impl Parser<$T>) -> Result<Self, ParseError> { .. }

Then, end-users could write their own implementation of a parser that could ignore invalid flags, or treat them as case-insensitive.

[KodrAus]

I've published a library that maintains the serde compatibility functions: https://github.com/KodrAus/bitflags-serde-legacy/

[KodrAus]

Last thing I think we should do before merging is run some benchmarks, just to make sure that the cost of parsing doesn't increase too badly with the number of flags.

[KodrAus]

Some very simple benchmarking suggests around 10ns parsing per flag on a flags type with 10 flags. There would be plenty of room to improve this, but it doesn't look like we've got a risk of exploding parsing time out.