HyKeywords are no longer an instances of string with a particular
prefix, but a completely separate object.
This means keywords no longer trip isinstance str checks, adding a
little bit of type safety to the compiler.
It also means that HyKeywords evaluate to themselves.
Closes#1352
This removes a lot of hy-reprs that were hard-coded into the hy-repr function itself. It also allows you to add a hy-repr for an existing class without monkey-patching the class.
