Re #571, allow raw tabs in basic and MLB strings

[eksortso]

In response to #571, these changes permit the horizontal tab character to be used in both basic strings and multi-line basic strings.

[eksortso]

This may take some investigation in case some parsers are doing this already, but perhaps this feature could be deferred until after the release of TOML 1.0?

[pradyunsg]

Yea, I don't see this as being super critical for 1.0 tbh. It's probably a good idea but be super conservative logic applies here.

[pradyunsg]

BTW, thanks for the ping on this; I'd completely forgotten about this PR.

[ChristianSi]

I oppose the delay. The problem stated in #571 is that the TOML spec is currently contradictory. The written spec says:

All other whitespace and newline characters remain intact.

Which, according to the common understanding of whitespace, allows tabs. But another sentence in the spec forbids tabs, and the ABNF forbids them as well. TOML 1.0 should not be ambiguous, hence this PR must be merged (or the ambiguity must be resolved in another way) before TOML 1.0 can be released.

[ChristianSi]

Also I just noted that the written spec currently seems to forbid line breaks within multi-line basic strings (they are within the forbidden range), which is of course silly. I added an inline comment to the PR on how this can be resolved.

This makes it all the more urgent to fix this (properly) before 1.0 is released.

[pradyunsg]

@ChristianSi consider me convinced.

[eksortso]

I oppose the delay. The problem stated in #571 is that the TOML spec is currently contradictory.

Thanks for reminding me. I'd forgotten that!