fix(Observable): use more granular Observable exports in compat mode

[bcherny]

Description: Use more granular exports in compat/Observable.ts to enable tree shaking optimization even if the consumer is compiling to CommonJS (instead of ES2015) modules.

Related issue (if exists): #3971

Importing Observable and calling Observable.create, with "module": "commonjs" vs "module": "es2015" in tsconfig.json:

Before patch

	"module": "commonjs"	"module": "es2015"
rxjs	79.6 KiB	14.7 KiB
rxjs-compat	80 KiB	80.1 KiB

After patch

	"module": "commonjs"	"module": "es2015"
rxjs	79.6 KiB	14.7 KiB
rxjs-compat	18.6 KiB	18.7 KiB

[coveralls]

Coverage increased (+0.2%) to 96.923% when pulling 01e3414 on bcherny:master into eba335b on ReactiveX:master.

[benlesh]

Great find, @bcherny!