Expose selector's result to allow adhoc caching

[jcready]

When a selector returns a complex value it can be difficult to get caching right. This will expose a way for selectors to see their own previous result which provides a way for users to do things like return the previous result if the result of the current call will be shallowly equal to previous result.

See #441 for an example of the feature request. Here is how this change would facilitate the solution:

import { createSelector } from 'reselect';
import { shallowEqualArrays } from 'shallow-equal';

const currentUsersPosts = createSelector(
  [state => state.currentUser.id, state=> state.posts],
  (userId, posts) => {
    const result = posts.filter(p=>p.id === userId);
    const lastResult = currentUsersPosts.lastResult();
    if (shallowEqualArrays(lastResult, result)) return lastResult;
    return result;
  }
);

[codecov]

Codecov Report

Merging #448 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #448   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           15        15           
=========================================
  Hits            15        15           

Impacted Files	Coverage Δ
src/index.js	100.00% <ø> (ø)

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[markerikson]

Superseded by #514 .