EVAL/EVALSHA support

This PR ads support for `EVAL`/`EVALSHA`. Please note that this is a very early PR to get feedback as soon as possible. A lot of stuff needs to be done. I will add a bunch of questions inline.

## TODOs
 - [ ] Error handling for non-scalar values in lists
 - [ ] Uneven number of elements in a list that should be treated like a hash
 - [ ] Non-float value as a hash value
 - [ ] Error handling for null
 - [ ] README
 - [ ] Logging
Or even completely put the validation code in the `execute` method and add a method like `getByStringOrThrow` to make it easy to get a parameter or throw an exception.