core: RxJS - switch from prototype-patched based operators to pipeable operators

[dsebastien]

Pipeable (also called "lettable") operators have been introduced in RxJS 5.5.0 and they provide a new way of composing observable chains.

Moreover, this new approach is more friendly with bundlers such as Webpack in order to automatically tree-shake the unused operators rather than being the developer manually taking care of not importing any operator that is not really needed.

We should move in this direction in order to get all the benefits of tree-shaking once we leverage it in Stark.

See https://blog.angularindepth.com/rxjs-understanding-lettable-operators-fe74dda186d3

Cfr

• https://github.com/ReactiveX/rxjs/blob/master/doc/pipeable-operators.md
• https://blog.angularindepth.com/rxjs-understanding-lettable-operators-fe74dda186d3

[dsebastien]

Comment from @christophercr :
Apparently there is an issue when using the pipeable operators together with Scope Hoisting enabled (see ReactiveX/rxjs#2981).
Perhaps we should wait until the new major version of RxJS (v 6.0) is released to see if this is solved

[dsebastien]

One more reason to move forward with this: ReactiveX/rxjs#3212

[dsebastien]

duplicate of #258