Ideas for API improvements

[titaniumtraveler]

I found a few instances where the winnow API could be nicer and instead of being slightly annoyed about it not being that, I thought I would rather document them here and see what other people think about these ideas.

1. ParserIterator should implement iterator on Self instead of &'a mut Self:
   This point is pretty important for me as it makes the API just nicer
   and has no apparent drawbacks.
   Especially because Iterator should almost always be implemented on Self,
   because the standard library already contains impl<T: Iterator> Iterator for &mut T,
   making an impl on &mut Self kind of redunant.
2. Similar to Iterator, Parser should have an impl<T: Parser> Parser for &mut Parser-impl as well.
   This can be useful when having a parser that contains some inner parser
   and enables just using the normal chaining functions on &mut inner_parser instead of needing to use .parse_next() manually.

Note that both changes might require major version bump,
though I'm not sure because of 1. the same impl should then be covered by the impl<T: Iterator> Iterator for &mut T of iterator
(though theoretically someone could depend on &mut &mut ParserIterator implementing Iterator.)
and for 2. it should not conflict with any existing code, but still might.

I will add to this list if I find more things, which could be improved.

[titaniumtraveler]

3. Parser::map
   should take FnMut instead of Fn, as it is much more useful.

[epage]

Looks like noms map supported this for a while (rust-bakery/nom#1202) but not Parser::map until rust-bakery/nom#1476 and we missed pulling this in.

As FnMut is a wider bound, this should be a safe change.

[epage]

Released in v0.5.27

[epage]

2. Similar to Iterator, Parser should have an impl<T: Parser> Parser for &mut Parser-impl as well.
This can be useful when having a parser that contains some inner parser
and enables just using the normal chaining functions on &mut inner_parser instead of needing to use .parse_next() manually.

This requires a blanket impl which conflicts with our existing blanket impl for FnMuts which allows any function to be a parser implicitly. We do offer Parser::by_ref to help with this.

[epage]

1. ParserIterator should implement iterator on Self instead of &'a mut Self:
   This point is pretty important for me as it makes the API just nicer
   and has no apparent drawbacks.
   Especially because Iterator should almost always be implemented on Self,
   because the standard library already contains impl<T: Iterator> Iterator for &mut T,
   making an impl on &mut Self kind of redunant.

This was initially reported against nom in rust-bakery/nom#1586 and then fixed in -bakery/nom#1587, after we forked.

I didn't initially fix it or copy over the fix due to lack of experience with iterator making it difficult for me to evaluate.

For example, iterator doesn't report errors in-line but requires you to access the returned value after iteration is complete. By supporting Iterator on ParserIterator, it will be even easier for people to overlook this, potentially leading to bugs.

[epage]

see also #349