Make OneOf structs readonly

[XanderStoffels]

Solves #120, improving performance when passing a struct to a method using the in keyword.

MS Docs
Blogpost with more insight
Stack overflow discussion

TL;DR

Using in for a parameter lets us pass by reference, but then prevents us from assigning a value to it. However the performance can actually become worse, because it creates a "defensive copy" of the struct, copying the whole thing. A way around this is to use read-only for a struct. When you pass it into an in parameter, the compiler sees that it's read-only and won't create the defensive copy, thereby making it the better alternative for performance.

[XanderStoffels]

On a side note, tests LeftSideFormatsWithCurrentCulture("en-NZ") and RightSideFormatsWithCurrentCulture("en-NZ") seem to be failing for me on the master branch before I introduced changes, so I left those alone. All other tests pass.

Edit: According to appveyor, all tests seem to pass. The tests that fail locally for me are datetime culture related, so Ill keep it at that.

[wdolek]

If I was in charge, I would approve and release, this is simple yet powerful change.