Implement chunkBy.
Implement binary predicate for chunkBy.
Move std.range.chunkBy to std.algorithm.groupBy.
Refactor to eliminate redundant call to r.empty.

.savePrev and .prev should be private.
Mark a unittest for std.file.thisExePath as safe
Merge pull request #2600 from tom-tan/safe-unittest-file-thisExePath

Mark a unittest for std.file.thisExePath as safe
Merge pull request #1453 from quickfur/chunkBy

Implement groupBy.