rlp: add functions for encoding

I'm reasonably confident that the encoding matches the output of
ethutil.Encode for values that it supports. Some of the tests have been
adpated from the Ethereum testing repository.

There are still TODOs in the code.

rlp/typecache.go

@@ -5,17 +5,20 @@ import (
 	"sync"
 )
 
-type decoder func(*Stream, reflect.Value) error
-
-type typeinfo struct {
-	decoder
-}
-
 var (
 	typeCacheMutex sync.RWMutex
 	typeCache      = make(map[reflect.Type]*typeinfo)
 )
 
+type typeinfo struct {
+	decoder
+	writer
+}
+
+type decoder func(*Stream, reflect.Value) error
+
+type writer func(reflect.Value, *encbuf) error
+
 func cachedTypeInfo(typ reflect.Type) (*typeinfo, error) {
 	typeCacheMutex.RLock()
 	info := typeCache[typ]
@@ -49,11 +52,27 @@ func cachedTypeInfo1(typ reflect.Type) (*typeinfo, error) {
 	return typeCache[typ], err
 }
 
+func structFields(typ reflect.Type) (fields []field, err error) {
+	for i := 0; i < typ.NumField(); i++ {
+		if f := typ.Field(i); f.PkgPath == "" { // exported
+			info, err := cachedTypeInfo1(f.Type)
+			if err != nil {
+				return nil, err
+			}
+			fields = append(fields, field{i, info})
+		}
+	}
+	return fields, nil
+}
+
 func genTypeInfo(typ reflect.Type) (info *typeinfo, err error) {
 	info = new(typeinfo)
 	if info.decoder, err = makeDecoder(typ); err != nil {
 		return nil, err
 	}
+	if info.writer, err = makeWriter(typ); err != nil {
+		return nil, err
+	}
 	return info, nil
 }