Add raw strings

[fuzzypixelz]

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See #1864.

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Add r%"..."% syntax for raw strings, where no escape sequences are recognized.

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[vi]

Maybe usual m%%%%%%%" ... "%%%%%%% (with enough per cent characters) can fill that niche (at least partially)?

[vi]

no escape sequences are recognized

But indentation works the same was as in usual multiline string or is also disabled (all bytes just copied literally)?

[yannham]

This is a good question. I would tend to disable indentation as well - so that raw is really raw, and there's no interpretation nor transformation whatsoever. But we could also imagine auto de-indenting them. I guess it depends on the use-case we see for raw strings.

You're right that in practice, multiline strings with long delimiters might be enough for most purposes.