add SpannedString, a String that remembers its span - #232
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In some situations it is useful to carry around span information along with a bare `String`. There isn't really a syn type that accepts either a `LitStr` or an `Ident` the way `String` does. Introduce `SpannedString`, which acts as that type and can be used as `ParseWrapper<SpannedString>`. One alternative to this is to implement `Deserialize` on `SpannedString` directly, but error handling for that would require a third side-channel thread local on top of the two we already have. This is much more straightforward implementation-wise. (It also has a pleasing cripness -- it's easier to explain.)
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In some situations it is useful to carry around span information along with a bare
String. There isn't really a syn type that accepts either aLitStror anIdentthe wayStringdoes. IntroduceSpannedString, which acts as that type and can be used asParseWrapper<SpannedString>.One alternative to this is to implement
DeserializeonSpannedStringdirectly, but error handling for that would require a third side-channel thread local on top of the two we already have. This is much more straightforward implementation-wise. (It also has a pleasing cripness -- it's easier to explain.)