Add YouDo.Holex and its test suite.
Add HolexError, and detection of missing keys.
Add detection of unused and duplicate keys.
Add custom errors in evaluation of Holexes.
Add parsing and parsing errors for Holexes.
Parse instances of Web.Scotty.Parsable (instead of Read).
Remove type parameter for custom errors from Holex evaluation.
Use Holex to parse YoudoID and YoudoVersionID from forms.
Add default values in Holexes.
Parse YoudoData via Holex.
Add `optional` combinator for Holex.
Parse YoudoUpdate via Holex.
Remove obsolete parameter parsing functions from YouDo.Web
Refactor functions for computing value of request.
Tweak implementation of bodyData
Simplify implementation of Holex as Functor.
Simplify implementation of Holex as Applicative.
Don't let default values suppress parsing errors.
Parse /0/youdos/:id using fromParams, not exceptiony `param`.
List Youdos, not just VersionIDs, from /versions