(web http): don't expose header-decl objects

(web http): don't expose header-decl objects

* module/web/http.scm: Change to not expose the header-decl objects,
  instead exposing header-parse, header-validator, header-writer et al.
  Explaining header decls in the manual was too complicated.
  (string->header, header->string): New helpers.
  (<header-decl>): Remove the `sym' field.
  (declare-header!): Adapt to header-decl change, and use
  string->header.
  (known-header?, header-parser, header-validator, header-writer): New
  procedures.

  Adapt to use the new procedures internally.