Add support for number ranges

[Ztry8]

This PR implements #601: support for number ranges in RON.

It's just syntactic sugar for structures with fields start, end or start, last.
It supports both std::ops and core::range.

Example:

let ranges = RangeTest {
    a: 0..5,
    b: 1..=3,
    c: 0.6..4.3,
    d: 0.3..=5.7,
};

let ser = ron::to_string(&ranges).unwrap();
assert_eq!(ser, "(a:0..5,b:1..=3,c:0.6..4.3,d:0.3..=5.7)");

let de: RangeTest = ron::from_str(&ser).unwrap();
assert_eq!(de, ranges);

[juntyr]

Thanks @Ztry8 for prototyping this feature!

[juntyr]

Also, this addition will need to be documented in the grammar

[Ztry8]

Thank you! I've already started working about this

[Ztry8]

@juntyr, is everything fine now? What should I put into CHANGELOG.md? Is the grammar.md correct?

[juntyr]

Thank you for the changes, I left some further comments

[Ztry8]

Thank you! l've already started to working on this

[Ztry8]

@juntyr, thanks for your help and support!

I’ve implemented all of your suggestions. Could you please check?

Sorry I didn’t commit some of your changes.
I had already pushed similar updates before seeing your comments

[juntyr]

It seems like you might need to refactor some imports from std to alloc

[juntyr]

Thank you @Ztry8 for your continued work! I've left some more comments

[Ztry8]

Thanks! l've already started to working on this

[Ztry8]

@juntyr, thank you for your cooperation!

I’ve implemented all of your suggestions. What do you think about them?