classify operator symbols like haskell-src-exts does

haskell-src-exts [classifies](https://github.com/haskell-suite/haskell-src-exts/blob/master/src/Language/Haskell/Exts/InternalLexer.hs#L340)  strings as either operator or identifier symbols with more complex rules than just `isSymbol`. `haskell-names` should do the same. Unfourtunately `isHSymbol` is not exported so has to be copied.