multiwayMerge/multiwayUnion review

[Ilya Yaroshenko]

Hi, I ported two this functions to mir-algorithm ~master: need 
them for work ASAP (License and Copyright are preserved). My 
small report:

  - Two algorithmic bugs in multiwayUnion: it is one line 
function, you can easily find them.
  - One truly significant algorithmic optimisation bug in 
multiwayMerge: see mir-algorithm ~master for fix.
  - Three kinds of bugs related to attributes and BetterC: ditto.

Sorry, can not fill the issues.

It is weird to duplicate Phobos code/API/functionality. Hope one 
day it will be split into DUB packages.

Best,
Ilya