Support Relation Iterator

[shmax]

With this change, you can now iterate over a Relation result:

$books = Book::all();

foreach($books as $book) {
  echo $book->name;
}

This is accomplished using generators and individual calls to PDO::fetch, so it can generally result in a memory savings for large result sets.

[codecov]

Codecov Report

Merging #65 (c1af770) into master (d59e14a) will increase coverage by 0.10%.
The diff coverage is 93.33%.

@@             Coverage Diff              @@
##             master      #65      +/-   ##
============================================
+ Coverage     95.33%   95.44%   +0.10%     
- Complexity      936      940       +4     
============================================
  Files            33       33              
  Lines          2272     2282      +10     
============================================
+ Hits           2166     2178      +12     
+ Misses          106      104       -2     

Files Changed	Coverage Δ
lib/Connection.php	91.56% <ø> (ø)
lib/Relation.php	92.85% <91.30%> (+2.01%)	⬆️
lib/Model.php	97.99% <100.00%> (+<0.01%)	⬆️
lib/Table.php	96.60% <100.00%> (ø)

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[shmax]

I tried valiantly to fix the issue with DatabaseTestCase not properly setting the default connection, but it exposed "write only" errors on the Sqlite tests in the CI, and I just couldn't figure out how to fix it. We can fight that battle in a different PR.

But in the interest of increasing code coverage enough to pass, I went ahead and implemented variadic lists on where and not...

Book::where("name = ?" , "Walden") // okay now

...which allowed me to remove some exceptions, and it's something we needed to do, anyway.