Don't use nullable types on primitive to avoid tons of (un)boxing

[pelmenstar1]

Type Int? in Kotlin JVM maps to reference type Integer. It causes memory allocations. In our case, nullability isn't necessary and can be avoided.

To save compatibility with the last version of the library, some variables types are explicitly marked.

Maybe we should break compatibility and then, boxed types won't be used at all.

[Droppers]

Looks good and in my opinion, it is okay to break compatibility for this, if you want to, of course, otherwise I will accept it as-is.

[pelmenstar1]

Conflicts fixed!