use const and let

[jimmywarting]

I was gona try to convert this to ESM but all the exports[xxxx]encode.bytes = x made it a bit weird.

So i just converted most of the code to use const and let, some default arguments here and there and some arrow fn and also reduced the number of exports by doing something like:

- exports.esds = {}
- exports.esds.encode = fn
- exports.esds.decode = fn
- exports.esds.encodingLength = fn

+ exports.esds = {
+  encode(...args) { ...}
+  decode(...args) { ...}
+  encodingLength(...args) { ...}
+ }

The conversion was mostly done with lebab. Didn't want to do too many changes - afraid to do any breaking stuff when there are no tests...
but the conversion to export an object instead makes it easier to iterate over to try and modernize it a bit furtherer

(have also updated all the dependencies)

[jimmywarting]

ping @jhiesey

[jhiesey]

Sorry, I haven't been maintaining this well recently. I'll take a look soon.