structtag

Closes #24. Per @fatih's note on the issue, the sentinel errors were unexported, so callers were stuck doing string-compare on the message to handle a 'tag not found' specifically. Promote them to \ and keep the lowercase names as aliases so the internal call sites compile unchanged. Lets users write: \\\

For #26. The current Get comment talks about an \ return value that doesn't exist (the function returns \) and mentions returning an 'empty string' which doesn't match how Tag is structured. Updated to actually describe the signature and the only error returned. This is intentionally separate from the export-sentinels PR (#27) so each can land on its own. Once #27 lands the doc can be tightened further to mention \.

Hi there! I tried using the library but got a little confused. Particularly the current documentation and API of was a matter of trial & error. The comment suggests a pattern like: In reality, is an error, not a boolean, so it wouldn't work. The main README adds to the confusion: Based on that panic I wouldn't expect the only error returned by is (which perfectly OK in my scenario). The panic example in the docs signals something more severe is going on. Moreover, because is not exported, callers can’t do something like A more sensible signature would be: so that the caller can check existence with a boolean rather than treating “not found” as an error. Similar to Go's type checking. Unfortunately, changing from to would break backwards compatibility. As an alternative, the library could introduce something like: that returns a boolean found flag instead of an error. This would leave Get unchanged (preserving compatibility) while providing a clearer, more “Go-idiomatic” lookup method. There's various possibilities here, not sure which is the best one. I figured I'd just share my first experience with this lib :-)