Abstract iterator binding to createIterator and callbacks to createCallback

[megawac]

Highlights

• Commit 1: simple optimization (jsperf) for many underscore methods common case with no context. Avoids calling .call() and instead calls the function directly whenever possible
• Commit 2 (see Add toLookupIterator to _.every, _.some, _.map, _.filter #1561): lookupIterator maps passed strings to _.property (see Avoid Function#call unless context is provided. #1557) and objects to _.matches allowing for some functionality thats existed in lodash for some time. It also allows _.find to cover _.findWhere, _.filter to cover _.where, _map to cover _.pluck and essentially adds _.rejectWhere as _.reject.

There are many side effects of this commit and so I can go grab dinner I'm going to pull examples from lodash docs (see included tests for more examples):

var characters = [
  { 'name': 'barney', 'age': 36, 'blocked': false },
  { 'name': 'fred',   'age': 40, 'blocked': true }
];

// using "_.pluck" callback shorthand
_.map(characters, 'name');
// → ['barney', 'fred']

// using "_.pluck" callback shorthand
_.filter(characters, 'blocked');
// → [{ 'name': 'fred', 'age': 40, 'blocked': true }]

// using "_.where" callback shorthand
_.filter(characters, { 'age': 36 });
// → [{ 'name': 'barney', 'age': 36, 'blocked': false }]

_.min(characters, 'age');
// → { 'name': 'barney', 'age': 36, 'blocked': false };

// using "_.where" callback shorthand
_.find(characters, { 'age': 40 });
// →  { 'name': 'fred',   'age': 40, 'blocked': true }

// using "_.where" callback shorthand
_.reject(characters, { 'age': 36 });
// → [{ 'name': 'fred', 'age': 40, 'blocked': true }]

[jashkenas]

@braddunbar -- When you have a free moment, I'd be curious to hear what you think of this patch...

[braddunbar]

I think this message goes on the following line (not as the context argument to _.find).

[braddunbar]

After mulling it over, I'm for this one. I like the property sugar and while I'm not particularly fond of the Function#call optimization there is precedent for it in Backbone.Events. 👍

[braddunbar]

I don't think we need this test as functions are just another object as far as _.where is concerned.

[megawac]

Maybe not, but its a gotcha I had in a previous commit (4512921) which is why I added a test for it

[braddunbar]

Fair enough! :)

[jdalton]

As #1609 noted that _.min and _.max should get this too.

[megawac]

Whoops, I forgot to add tests for max and min but they both will benefit from lookup iterator. I'll do that right quick thanks

[jashkenas]

Mind rebasing this so that it merges in cleanly?

[megawac]

@jashkenas rebase is done