Tighter loops on Enumerable.Min, .Max & .Average.

Fixes #2238.

Changes the flow of these aggregate operations to use tighter loops.

A test of the performance changes can be see at
https://github.com/hackcraft/Enumerable-Tester/blob/master/Enumerable%20Aggregate%20Performance.ods?raw=true

Notably:

1. Most tested uses showed an increase in performance, sometimes dramatically.
2. The only cases with a decrease in performance greater than 5% were in short
paths (e.g. single-element sequences) and so the total performance impact is
slight.
3. For other cases that showed a decrease in performance there were similar
cases (same operation on same datatype) with an increase in performance either
comparable or overwhelming the decrease.

Merge branch 'master' into faster_linq_aggregates

Remove extraneous while.

Simplify while loops on nullable types.

Merge branch 'master' into faster_linq_aggregates

Author: JonHanna