Move Symbol shims into single file.

[benlesh]

Considering we're shimming Symbol and items on it frequently, I think it would be best to move it into a single file.

That would make this:

import {$$iterator} from '../utils/Symbol_iterator';
import {$$observable} from '../utils/Symbol_observable';

const something = {
  [$$iterator]() { }
  [$$observable]() { }
};

... into this...

import {Symbol} from '../utils/Symbol';

const something = {
  [Symbol.iterator]() { }
  [Symbol.observable]() { }
};

[kwonoj]

Typesciprt side support of es6 global symbol (microsoft/TypeScript#980) makes bit hard to use module name Symbol, causes name collision in es6 build like

192   [Symbol.iterator]() {                                                                                                
       ~~~~~~                                                                                                                                                                                                                                         
src/operators/zip-support.ts(192,4): error TS2470: 'Symbol' reference does not refer to the global Symbol constructor object.                                                                                                                         

while other (cjs, etcs) are built. What about having different name for this module, maybe SymbolPolyfill or something else?

p.s :
seems global symbol is supported for es6 only, using those global symbol and targeting es5 will simply raise lookup failure.

161   [Symbol.iterator]() {
       ~~~~~~
src/operators/zip-support.ts(161,4): error TS2304: Cannot find name 'Symbol'.

[benlesh]

Sigh. TypeScript. LOL. We may have to go back to using Babel to target ES5.

TypeScript also doesn't allow generators in code targeting ES5. Which isn't a big issue so far, but I've hit it playing around.