Use raw literal for string formatting

[dsnet]

When formatting strings, avoid using strconv.Quote if the string looks
like it has already been escaped. Instead, use a raw string literal
by wrapping the string with '`' characters if possible.
For now, we still use strconv.Quote if the input string contains newlines
to maintain the property that Format outputs a single line.

Also, prefix strings obtained by the Stringer.String method with a 's'.
This allows users to more easily distinguish when an output really is
of type string or if the String method was used.

[neild]

I note that the s"string" syntax is non-standard and will probably result in a fair bit of user confusion.

[dsnet]

I couldn't think of anything better. The fundamental problem is that Stringer.String is an irreversible transform. I considered using String() = as the prefix, but that looks overly verbose when formatting map keys or a slice of elements, each with a String method.

Regardless, I think some prefix is better than no prefix. The previous output was subtly lying to the user. We could avoid the Stringer.String altogether, but I believe there is value is having user defined outputs.

[neild]

Yeah, I don't have a better alternative, and I agree that marking Stringer output is worthwhile. It's still going to cause confusion, though.

[dsnet]

Submitting as is then. We can always adjust this in the future if someone has a better idea.