Restructuring: Split up operators and syntax

[dwestheide]

Currently, all operators are defined in one big trait, and the same is true for the syntax. Also, operators for RDD and Stream are created as anonymous classes, making it more complicated than necessary to extend the library with yet unsupported operators.

Restructure the operators and syntax into separate traits and files that can then also easily be extended with custom operators without too much hassle.

This is a breaking change, which should be done rather now than later.