Chunked transfer encoding

HTTP/1.1 supports chunked transfer encoding: the body is a series of chunks, each prefixed by its size in hexadecimal followed by CRLF, then the chunk data and a trailing CRLF. A chunk-size line may carry optional chunk extensions after a ';' which you must ignore. The body terminates with a zero-size chunk ('0\r\n\r\n'). Given the raw chunked body string, return the decoded message. Sizes are valid hex; the body is well-formed.