Refactor Stream/Events generics

[krzysztofreczek]

Here is the PoC for a little bit of changed implementation of Stream/Events relation.

What I wanted to achieve:

• Allow library users to use the straightforward genetic type definition Stream[Foo] / Events[Foo] instead of Stream[*Foo], and use pointers to the generic type only when it is necessary.
• Hide the complexity and API of library internals like PopChanges, Load... etc., and do not require the user to know how to implement that properly.
• Simplify the reflection-based logic and stream instantiation. Now we have a pretty convenient stream.New[T](eq Events[T]) T
• Simplify public library functions. Now we have: stream.Record[T](*T, e Event[T]) error

[krzysztofreczek]

@m110, I found some of the documentation difficult to maintain. I moved the interface method docs closer to the signatures. https://github.com/ThreeDotsLabs/esja/pull/14/commits/16cb12d4ace17dba7cbfb9aa19883783112aa196