eth/tracers/js: avoid compiling js bigint when not needed (#30640)

While looking at some mem profiles from `evm` runs, I noticed that
`goja` compilation of the bigint library was present. The bigint library
compilation happens in a package `init`, whenever the package
`eth/tracers/js` is loaded. This PR changes it to load lazily when
needed.

It becomes slightly faster with this change, and slightly less alloc:y. 

Non-scientific benchmark with 100 executions: 
```
time for i in {1..100}; do ./evm --code 6040 run; done;
 ```

current `master`:

```
real    0m6.634s
user    0m5.213s
sys     0m2.277s
```
Without compiling bigint
```
real    0m5.802s
user    0m4.191s
sys     0m1.965s
```

Author: holiman

eth/tracers/js/goja.go

@@ -22,6 +22,7 @@ import (
 	"fmt"
 	"math/big"
 	"slices"
+	"sync"
 
 	"github.com/dop251/goja"
 	"github.com/ethereum/go-ethereum/core/tracing"
@@ -59,9 +60,17 @@ func init() {
 	tracers.DefaultDirectory.RegisterJSEval(newJsTracer)
 }
 
-// bigIntProgram is compiled once and the exported function mostly invoked to convert
-// hex strings into big ints.
-var bigIntProgram = goja.MustCompile("bigInt", bigIntegerJS, false)
+var compiledBigInt *goja.Program
+var compileOnce sync.Once
+
+// getBigIntProgram compiles the bigint library, if needed, and returns the compiled
+// goja program.
+func getBigIntProgram() *goja.Program {
+	compileOnce.Do(func() {
+		compiledBigInt = goja.MustCompile("bigInt", bigIntegerJS, false)
+	})
+	return compiledBigInt
+}
 
 type toBigFn = func(vm *goja.Runtime, val string) (goja.Value, error)
 type toBufFn = func(vm *goja.Runtime, val []byte) (goja.Value, error)
@@ -567,7 +576,7 @@ func (t *jsTracer) setBuiltinFunctions() {
 func (t *jsTracer) setTypeConverters() error {
 	// Inject bigint logic.
 	// TODO: To be replaced after goja adds support for native JS bigint.
-	toBigCode, err := t.vm.RunProgram(bigIntProgram)
+	toBigCode, err := t.vm.RunProgram(getBigIntProgram())
 	if err != nil {
 		return err
 	}