2b0eb969

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Add autodocodec-http-api-data for form-urlencoded data

A type with a HasObjectCodec instance could not be posted as
application/x-www-form-urlencoded without a second, hand-written
description of its fields. http-api-data's own generic derivation is no
substitute: it derives names from Haskell record fields via a label
modifier, so it would be a third independent spelling of the same field
names rather than a shared one.

This is autodocodec-servant-multipart's interpreter retargeted at
Web.FormUrlEncoded.Form, which is likewise a flat map from text keys to
text values. A Form's HashMap Text [Text] models repeated keys natively,
so ArrayOfCodec fits it better than it fits multipart's flat list of
inputs, and unlike the multipart spec every corpus type round-trips,
Example included.

Three places where copying multipart verbatim would have been wrong:

* Form derives Semigroup from HashMap, whose union is left-biased and so
  would silently drop the second form's values under a shared key, where
  multipart's mappend concatenates. Hence unionForm.

* Booleans encode lower case, to match toUrlPiece @Bool and the HTML form
  convention, so that a value survives http-api-data's own parser. The
  decoder still accepts either spelling.

* A field is absent exactly when its key is absent, which is lossless.
  dropEmptyFormValues is exported separately, for callers who need a
  shell's unset variable to read as absence rather than as the empty
  string.

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