Adding load(fname) and save(fname) - save being really eval_file(fname)

[ldemailly]

Fixes #38
Fixes #125

tests have been manual, loading all the examples, saving, reloading comparing (how I stepped on #125)
not exactly sure how to ... actually test this

constants if not exact might cause an issue

[ldemailly]

because I moved some code from one file (eval.go) to another (eval_api.go) it's probably easier to review individual commits

[ccoVeille]

Does this method must comply with an interface?

You should add a comment then, because the unused string is awkward

[ldemailly]

yeah it's defined a bit above

	jsonFn.Callback = saveFunc // save to file.

which is

// ExtFunction is the signature of what grol will call when the extension is invoked.
// Incoming arguments are validated for type and number of arguments based on [Extension].
// eval is the opaque state passed from the interpreter, it can be used with eval.Eval etc
// name is the function name as registered under.
type ExtFunction func(eval any, name string, args []Object) Object

like everything in the file, in short it doesn't need to know its own name

[ccoVeille]

I might be missing something but either you add a comment explaining why you need n

or you do that

Suggested change

	n := 0
	for _, k := range keys {
	for n, k := range keys {

Then return n+1 at the end of the function

[ldemailly]

n would be out of scope...

[ldemailly]

I can do var k string though and drop the := though it seems icky to use loop vars outside of the loop

[ldemailly]

but that'd be incorrect for empty keys

[ldemailly]

so in short n counts the number of succcesful writes, it happens to be len(keys) if there are no errors but can be shorter if there is an error.