Reimplement elemMatch

[miripiruni]

For #135

/cc @zxqfox @tadatuta @indutny

[qfox]

Here is the problem with back-compat:

block('b').def()(
  elemMatch(function(){ return this.elem === 'e1'; })('1'),
  elem('e2')('2')
);
// +
{ block: 'b', elem: 'e2' }

[miripiruni]

//cc @veged

[miripiruni]

@zxqfox Here is two tests about priority.

[qfox]

Actually, I don't get why?

[arikon]

We decided to remove elemMatch() in 5.0 in favor of

• elem('*').match(); and document pequilarity
• elems('a', 'b', 'c', ...)

[qfox]

Don't forget to remove or fix elem('*')