Override first, last and single on RealmResults

[nielsenko]

Avoid instantiating a _RealmResultsIterator for first and
single calls, and avoid iterating the entire collection for last.

[coveralls]

Pull Request Test Coverage Report for Build 5345350646

• 14 of 15 (93.33%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.008%) to 88.633%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/src/results.dart	14	15	93.33%

Files with Coverage Reduction	New Missed Lines	%
lib/src/subscription.dart	1	90.48%

Totals
Change from base Build 5344969691:	-0.008%
Covered Lines:	3236
Relevant Lines:	3651

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💛 - Coveralls

[blagoev]

added some suggestions on the exceptions mostly. I think we should be doing these optimizations.