Reduce: Use Adiar's I/O Wrappers

[SSoelvsten]

In the reduce algorithm, all mappings due to the first reduction rule are placed in a tpie::file_stream<mapping>. With #397 we have cleaned up Adiar's wrappers on files and streams and should use these instead (especially to support #199 later).

• [ ] Replace the tpie::file_stream<mapping> in __reduce_level using Adiar's wrappers: file<mapping>, file_writer<mapping>, and file_stream<mapping>.