Reduce marshal and unmarshal overhead (#1044)

* Reduce marshal and unmarshal overhead

Targeted optimizations to reduce performance overhead introduced by
recent feature additions and the unsafe removal.

Unmarshal:
- parseKeyval: access the node directly in the builder's slice to set
  Raw, bypassing NodeAt which triggers a GC write barrier for the
  nodes-pointer on every key-value expression.
- Iterator.Next: cache the *nodes slice dereference in a local variable
  to avoid repeated pointer-to-slice indirection in the hot loop.

Marshal:
- Guard shouldOmitZero calls with an inlineable options.omitzero check.
  shouldOmitZero has inlining cost 1145 (budget 80), so avoiding the
  function call when omitzero is not set removes per-field overhead.
- Inline the isNil check in encodeMap. isNil has inlining cost 93
  (budget 80), so expanding it at the single hot call site avoids
  per-map-entry function call overhead.

Update README benchmarks.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

unstable/parser.go

@@ -363,9 +363,10 @@ func (p *Parser) parseKeyval(b []byte) (reference, []byte, error) {
 	p.builder.Chain(valRef, key)
 	p.builder.AttachChild(ref, valRef)
 
-	// Set Raw to span the entire key-value expression
-	node := p.builder.NodeAt(ref)
-	node.Raw = p.rangeOfToken(startB[:len(startB)-len(b)], b)
+	// Set Raw to span the entire key-value expression.
+	// Access the node directly in the slice to avoid the write barrier
+	// that NodeAt's nodes-pointer setup would trigger.
+	p.builder.tree.nodes[ref].Raw = p.rangeOfToken(startB[:len(startB)-len(b)], b)
 
 	return ref, b, err
 }