Brings back transformer (#891)

[paualarco]

Brings back transform signature and type Transformer[+A, -B] = Observable[A] => Observable[B].

Some extra motivations of this PR are that the current monix-dynamodb already exposes this operator in its api, and it is also shown in the documentation, which could create confusions.

[Avasil]

I'm wondering where to put the alias - Observable companion object or reactive package object

[paualarco]

I'm wondering where to put the alias - Observable companion object or reactive package object

It might be more intuitive to put it in the reactive package.
And that should also apply to Operator[-I, +O], don't you think?

[Avasil]

I think it's fine for Operator because it is mostly used internally anyway and moving it would break compatibility

[paualarco]

I'm wondering where to put the alias - Observable companion object or reactive package object

Will address the change whenever there is a final decision :-)