rlp, trie: faster trie node encoding (#24126)

This change speeds up trie hashing and all other activities that require
RLP encoding of trie nodes by approximately 20%. The speedup is achieved by
avoiding reflection overhead during node encoding.

The interface type trie.node now contains a method 'encode' that works with
rlp.EncoderBuffer. Management of EncoderBuffers is left to calling code.
trie.hasher, which is pooled to avoid allocations, now maintains an
EncoderBuffer. This means memory resources related to trie node encoding
are tied to the hasher pool.

Co-authored-by: Felix Lange <fjl@twurst.com>

trie/committer.go

@@ -44,7 +44,6 @@ type leaf struct {
 // By 'some level' of parallelism, it's still the case that all leaves will be
 // processed sequentially - onleaf will never be called in parallel or out of order.
 type committer struct {
-	tmp sliceBuffer
 	sha crypto.KeccakState
 
 	onleaf LeafCallback
@@ -55,7 +54,6 @@ type committer struct {
 var committerPool = sync.Pool{
 	New: func() interface{} {
 		return &committer{
-			tmp: make(sliceBuffer, 0, 550), // cap is as large as a full fullNode.
 			sha: sha3.NewLegacyKeccak256().(crypto.KeccakState),
 		}
 	},