Upgraded ByteballCore to es6 using lebab

[jamesdwilson]

No description provided.

[fishera123]

I think would be great if we also add eslint to the project.

[jamesdwilson]

@fishera123 Do you think that should happen in another PR for purity?
This is the command I used to convert, and anyone else can verify this. If we introduced eslint at this stage, it might muddy the transparency a bit.
lebab --replace ./byteballcore --transform arrow,for-of,for-each,arg-rest,arg-spread,obj-method,obj-shorthand,no-strict,multi-var,let,class,template,default-param,destruct-param

[fishera123]

@jamesdwilson agree with you it can be introduced in another PR.

[tonyofbyteball]

what's the point?