[#24] Add union and unionWith

[chshersh]

Resolves #24

Unfortunately, implementation uses containers package so it won't be to easy to remove containers from dependencies... It's possible, but not trivial. But looks like at least we don't need vector after PR #28 by @vrom911.

[int-index]

OK, let's keep using containers for now. At some point in the future let's try to rewrite this to something more efficient:

1. Allocate new mutable vectors of length n+m.
2. Traverse input vectors simultaneously, copying elements in the right order. We should be able to take advantage of the fact that the elements are in a particular order. For regular sorted vectors it's very simple, for our cache-friendly order it can be tricky, but should be possible (I hope)
3. While doing this, count the actual amount of elements copied (it can be less than n+m due to conflicts). Shrink the mutable vector.
4. Freeze and return.

[chshersh]

@int-index Thanks for feedback! I've created issue #30 using your comment.