Use constants for scoped variables in JS functions

[nesk]

Currently, the scoped variables in JS functions are assignable. There is no reason to allow this behavior, the const keyword should used instead of var in scoped variable declarations:

https://github.com/nesk/rialto/blob/21a21181249ca0c09268c84fc93e2650849a37f1/src/node-process/Data/Unserializer.js#L48