Add IndexBy

This is a squashed merge of PR #562 that closes #560.

Fix ToDataTable to skip write-only properties

This is a squashed merge of PR #567 that fixes #564.

Use List for Memoize's cache (performance)

Indexing into an IList<> requires an interface call, and can't be
inlined. Since the cache field is private, there is no reason to
not type it instead as List<>, thus get a free performance gain at
no cost.

Closes #546

Document type parameter of IExtremaEnumerable<> [ci skip]

Add AwaitCompletion for await “tasks”

Merge of PR #479

Reuse PartialCount more generally in count-methods

Bump version to 2.10.0

Update test project to enable Live Unit Testing in VS 2017

The Live Unit Testing (LUT) feature was introduced in Visual Studio
2017 Enterprise Edition version 15.3.

NuGet packages added to the test project:

- NUnit3TestAdapter:
  Visual Studio needs it for LUT to find the tests

- Microsoft.NET.Test.Sdk:
  LUT asked if it was installed thus assuming it is required
  
It was also necessary to explicitly set the startup object of the test
project to MoreLinq.Test.Program because of additional possible start
points created by one or other of the NuGet packages above.

FullGroupJoin TSecond type param doc typo; fixes #349

List From in README & package description