Avoid Collision with user environment variables

[MichaReiser]

The extracted environment variables might collide with variables defined by the user:

const x = 100;

parallel.from([1, 2, 3])
  .inEnvironment({ x: 10 })
  .map((value, env) => value * x * env.x);

The easiest approach is to use "ugly" names for the variables extracted in the environment hash. E.g. _______parallel-es-extracted-closure-variable________x. This should reduce collisions to a minimum.