refactor: cleaner header parsing

Builds off seanmonstar#134 (swar), seanmonstar#138 (Bytes cursor)

Cleaner, faster and less macros !

- Broke down header-parsing into clean conceptual steps (whilst being faster !)
- Added InnerResult allowing for idiomatic `?` early-exits, removing need for parsing-helper macros
- Removed macros.rs, leaving only `byte_map!` (response header-parsing macros should become functions)

benches/parse.rs

@@ -152,7 +152,11 @@ fn version(c: &mut Criterion) {
         .bench_function(name, |b| b.iter(|| {
             black_box({
                 let mut b = httparse::_benchable::Bytes::new(input);
-                httparse::_benchable::parse_version(&mut b).unwrap()
+                match httparse::_benchable::parse_version(&mut b) {
+                    // Somewhat awkward, but this is internal code so it's ok.
+                    Ok(_) | Err(None) => (),
+                    Err(Some(e)) => panic!("parse_version failed: {}", e),
+                }
             });
         }));
     }