Add \xHH Unicode escape code to basic strings

[eksortso]

Per a conversation in #622, this PR adds a two-character \x escape code sequence for the first 256 Unicode code points. This provides a short explicit sequence for, among other code points: ASCII characters, certain control characters, and whitespace characters such as tab (\x09) and space (\x20) that need visible representation without resorting to longer \u and \U codes. Such sequences are commonly found in languages that also support the \u and \U codes for Unicode.

This PR replaces #709 and is intended for a post-v1.0.0 version of the TOML standard.

[marzer]

@pradyunsg Where did we land with this? It seems like a friction-free addition to the language (and one that would bring TOML in line with many other languages which also support \x). It's also practically free for implementers since the code paths that would already be handling \u and \U can be trivially extended to support \x.

[pradyunsg]

Yea, let's do this.

[marzer]

@eksortso Do you have time to update this PR against the current master? If not I'm happy to fork your changes and look after it.

[eksortso]

I'll update it later this evening.

[marzer]

Awesome, thanks @eksortso

[eksortso]

@marzer I think that's got it.

[eksortso]

The second change to CHANGELOG.md dresses up the bare URL for toml.io, just to appease the linter. I'll undo it if necessary.

[eksortso]

For some reason I'd thought this had already been taken care of. It's up to date again.

@marzer @pradyunsg Care to take look at it?

[ChristianSi]

Looks good otherwise!

[eksortso]

@pradyunsg Any updates on this? Is the issue with pre-commit.ci impeding merges? Or are you planning a larger roadmap for the next release of TOML?

[pradyunsg]

Thanks @eksortso! ^>^