(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.