Optimize solidity mapping perf

[otherview]

Description

This PR optimizes the generic Mapping[K, V] implementation by cutting down on allocations and CPU overhead in both Get and Set:

• Pooled RLP buffers/readers via sync.Pool to avoid per-call allocations of bytes.Buffer and bytes.Reader.
• Fixed double-encode bug in Set (now returns the pooled encoding instead of calling rlp.EncodeToBytes twice).
• Nil-handling in Get now only does a single reflection check on missing keys.

---

Benchmark Comparison

Case	Old	New	Δ (ns/op)	Δ (allocs)	Δ (bytes)
Set	677 ns/op · 16 allocs · 475 B/op	550 ns/op · 11 allocs · 344 B/op	–127 ns (~19%)	–5	–131 B
GetEmpty	322 ns/op · 5 allocs · 168 B/op	313 ns/op · 5 allocs · 168 B/op	–9 ns	0	0
GetNonEmpty	540 ns/op · 11 allocs · 416 B/op	535 ns/op · 10 allocs · 368 B/op	–5 ns	–1	–48 B

---

Benchmark test

// mapping_bench_test.go
package solidity

import (
	"github.com/vechain/thor/v2/muxdb"
	"github.com/vechain/thor/v2/state"
	"github.com/vechain/thor/v2/trie"
	"math/big"
	"testing"

	"github.com/vechain/thor/v2/thor"
)

// stringKey implements the Key interface
type stringKey string

func (k stringKey) Bytes() []byte { return []byte(k) }

// ComplexVal is a sample value type containing *big.Int, uint32, and thor.Address
type ComplexVal struct {
	N *big.Int
	U uint32
	A thor.Address
}

func BenchmarkMapping(b *testing.B) {
	// 1) spin up an in-memory state.State (so you don’t hit disk or a real trie DB)
	//    adjust these calls to whatever the real constructor is
	st := state.New(muxdb.NewMem(), trie.Root{})

	// 2) you can pass `nil` for the charger if you don’t care about gas accounting
	ctx := NewContext(thor.Address{}, st, nil)

	m := NewMapping[stringKey, ComplexVal](ctx, thor.Bytes32{})

	key := stringKey("bench")
	missingKey := stringKey("missing-key")
	val := ComplexVal{}

	b.Run("Set", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			if err := m.Set(key, val, true); err != nil {
				b.Fatal(err)
			}
		}
	})

	b.Run("GetEmpty", func(b *testing.B) {
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			if _, err := m.Get(missingKey); err != nil {
				b.Fatal(err)
			}
		}
	})

	// preload
	if err := m.Set(key, val, true); err != nil {
		b.Fatal(err)
	}

	b.Run("Get", func(b *testing.B) {
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			if _, err := m.Get(key); err != nil {
				b.Fatal(err)
			}
		}
	})
}

Fixes # (issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.38095% with 30 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/hayabusa@e9bed70). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
builtin/staker/aggregation/service.go	0.00%	12 Missing ⚠️
builtin/staker/validation/repository.go	18.18%	8 Missing and 1 partial ⚠️
builtin/solidity/mapping.go	86.11%	4 Missing and 1 partial ⚠️
builtin/staker/delegation/service.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##             release/hayabusa    #1221   +/-   ##
===================================================
  Coverage                    ?   58.34%           
===================================================
  Files                       ?      273           
  Lines                       ?    28978           
  Branches                    ?        0           
===================================================
  Hits                        ?    16907           
  Misses                      ?    10627           
  Partials                    ?     1444           

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[paologalligit]

Looks good, left just a minor comment you can ignore