type decode = [ | `Field of Field.field Location.with_location| `Await| `End of string| `Malformed of string ]val decoder : Field.witness Field_name.Map.t -> decoderdecoder parsers returns a decoder with a set of field-name binded with expected kind of value. When the decoder will decode value of a given field-name, it will try to extract expected value describe by the given Field.witness or it returns a Unstructured.t.
decode decoder is:
`Field field when extracted a field.`Await when the decoder wants more input.`End trailer when we reach end of the header. trailer is what the user gaves which is not the part of the header.`Malformed err we we reach an error while decoding.