Make MemoryLru tree-shakeable

[schmidt-sebastian]

Our hard-coded dependency means that tree-shaking doesn't remove the unused LRU code for memory persistence. This PR removes the dependency and should also completely remove the estimated byte size accounting in the field value rewrite.

Before:
317762 firebase-firestore.js
274464 firebase-firestore.memory.js

After:
315861 firebase-firestore.js
270591 firebase-firestore.memory.js

[wu-hui]

I know very little about tree-shaking, can you explain a bit why removing static factory methods helps?

LGTM still.

[schmidt-sebastian]

My five second summary of tree shaking: Our build pipeline drops unused imports and top-level exports. So if you have:

class A {}
class B {}
const b = B();

A won't be in the final bundle. If instead, you have:

class A {}
class B {  
 function unused() {
  return new A();
 }
}
const b = B();

RollUp doesn't know whether you are actually using B.unused() and hence A (since there is no guarantee for strong typings). In this snippet, A and B are included in the final bundle.