Change DDoc examples in std.algorithm to documented unittests

Examples with text succeeding them that is not associated with
another example are left as-is.
Fix std.algorithm.splitter example
Fix std.algorithm.find examples
Fix std.algorithm.findAdjacent example
Fix std.algorithm.partition3 example
Fix std.algorithm.topN example
Fix std.algorithm.setIntersection documentation
Remove extraneous header from std.algorithm.reduce example
Change std.algorithm.map example to not use I/O
Merge pull request #1778 from JakobOvrum/algorithmexamples

Change DDoc examples in std.algorithm to documented unittests