Preserve spans of all punctuation and keyword tokens in ensure #198
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This uses a technique that has worked well in
serde_json::json!
— we keep a double copy of the input tokens where we can match punctuation and keywords against one copy and capture the other copy as $:tt.Relevant for compiler error messages that want to point to punctuation or keyword tokens; without this change the compiler would see the output tokens as originating from inside the macro implementation, rather than what was written by the user, resulting in confusing placement of diagnostics.