feat: update to rxjs 5.5.0 and switch to lettable operators

Bumps the required RxJS version to 5.5.0 and gets rid of our RxChain in favor of using the lettable Rx operators. Refactors all the usages and updates the configs.

Fixes #7275.

Author: crisbeto

.github/CODEOWNERS

@@ -48,7 +48,6 @@ src/lib/core/datetime/**           @mmalerba
 
 # CDK
 src/cdk/coercion/**                @jelbourn
-src/cdk/rxjs/**                    @jelbourn
 src/cdk/observers/**               @jelbourn @crisbeto
 src/cdk/collections/**             @jelbourn @crisbeto @andrewseguin
 src/cdk/a11y/**                    @jelbourn @devversion

CODING_STANDARDS.md

@@ -127,33 +127,20 @@ class ConfigBuilder {
 ```
 
 #### RxJS
-When dealing with RxJS operators, import the operator functions directly (e.g.
-`import "rxjs/operator/map"`), as opposed to using the "patch" imports which pollute the user's
-global Observable object (e.g. `import "rxjs/add/operator/map"`):
+When dealing with RxJS operators, import the lettable operator (e.g.
+`import {map} from 'rxjs/operators'`), as opposed to using the "patch" imports which pollute the
+user's global Observable object (e.g. `import 'rxjs/add/operator/map'`):
 
 ```ts
 // NO
 import 'rxjs/add/operator/map';
 someObservable.map(...).subscribe(...);
 
 // YES
-import {map} from 'rxjs/operator/map';
-map.call(someObservable, ...).subscribe(...);
+import {map} from 'rxjs/operators';
+someObservable.pipe(map(...)).subscribe(...);
 ```
 
-Note that this approach can be inflexible when dealing with long chains of operators. You can use
-the `RxChain` class to help with it:
-
-```ts
-// Before
-someObservable.filter(...).map(...).do(...);
-
-// After
-RxChain.from(someObservable).call(filter, ...).call(map, ...).call(do, ...).subscribe(...);
-```
-Note that not all operators are available via the `RxChain`. If the operator that you need isn't
-declared, you can add it to `/core/rxjs/rx-operators.ts`.
-
 #### Access modifiers
 * Omit the `public` keyword as it is the default behavior.
 * Use `private` when appropriate and possible, prefixing the name with an underscore.