JSON prebuild export and import broken?

## *What* is wrong?
To make the loading time faster, I thought to prebuild my kernels once as JSON with toJSON(), and use this JSON to create my MapKernels. But the toJSON() is out of date (not handling native functions correct), and also the intern fromJSON(...) do not work. I tried to patch them, and now I can export, but I don't get the import working.
Additional information: I use a map function with some map return values, and custom native functions.

## *Where* does it happen?
Not specific to a system.

## *How* do we replicate the issue?
- Create a kernel with createKernelMap(...)
- Use at least one map function
- Use at least one native function
- Export the kernel with toJSON()
- Try to create a new kernel with that JSON.

## *How* important is this (1-5)?
2

## Expected behavior (i.e. solution)
Should work

## Other Comments
After some tests to patch fromJSON(...) I got it working, but the compiled fragment shader complains about complains about the missing map result variables.
Here are my patched functions:
```
toJSON = function() {
return this.traceFunctionCalls(this.rootNode.name).reverse().map(name => {
const nativeIndex = this.nativeFunctionNames.indexOf(name);
if (nativeIndex > -1) {
return {
name,
source: this.nativeFunctions[nativeIndex].source
};
} else if (this.functionMap[name]) {
return this.functionMap[name].toJSON();
} else {
throw new Error(`function ${ name } not found`);
}
});
}
```
```
fromJSON = function(jsonFunctionNodes, FunctionNode) {
this.functionMap = {};
for (let i = 0; i < jsonFunctionNodes.length; i++) {
const jsonFunctionNode = jsonFunctionNodes[i];
if (jsonFunctionNode.settings) {
const functionNode = new FunctionNode(jsonFunctionNode.ast, jsonFunctionNode.settings);
functionNode.triggerImplyArgumentType = this.assignArgumentType.bind(this);
// For some reason the FunctionNode object is missing some members, which I found in the FunctionBuilder.
// I'm not sure, if this is correct, but it works somehow.
for (const key of Object.keys(functionNode)) {
if (functionNode[key] === null && this[key] && typeof this[key] === 'function') {
functionNode[key] = this[key].bind(this);
}
}
this.addFunctionNode(functionNode);
if (functionNode.isSubKernel) {
this.subKernelNodes.push(functionNode);
}
this.functionNodes.push(functionNode);
} else {
this.nativeFunctions.push(jsonFunctionNode);
}
}
return this;
};
```

Inizia con createKernelMap e i punti di ingresso toJSON/fromJSON, quindi esamina la gestione di FunctionNode per le funzioni map e le funzioni native. Riproduci la sequenza di esportazione/importazione descritta nell’issue e traccia l’output del fragment shader compilato. L’attività è completata quando un kernel contenente valori restituiti da map e funzioni native personalizzate può essere esportato, importato e compilato senza variabili dei risultati di map mancanti.

Scritto dal modello di indicizzazione a partire dal testo della issue.