Update to slice 0.7
Remove extra newline
Improve aggregation framework

* Introduce a compiler for accumulators
* Introduce compiler for aggregation functions
* Add annotations, so that aggregation functions can be defined as a combination of static methods
* Benchmarks show ~10% performance improvement on TPCH query 1, and ~30% speed up for SUM()
Remove AbstractSimpleAggregationFunction
Add unit test for evaluate_classifier_predictions
Add support for aggregation functions with multiple arguments
Add support for overloaded aggregations in compiler
Remove broken approx_distinct(boolean) function
Refactor approx_distinct to use new aggregation framework
Refactor COUNT_IF
Refactor approx_set
Refactor count aggregation
Rename AggregationFunctionMetadata
Add support for approximate aggregations in compiler
Refactor approximate sum aggregations
Refactor approximate average aggregations
Refactor approx_percentile
Refactor custom_sum
Clean up aggregations

* Add name and approximate to InternalAggregationFunction
* Migrate variance/stddev to new aggregation framework
* Remove a bunch of dead code