xunit/xunit#2123: Analyzer to convert Assert.Collection to Assert.Single

[etherfield]

Resolves #2123

[etherfield]

I've added support for expression lambdas, statement lambdas, and parameter-methods as these cases are the most commonly used.

Assert.Collection(collection, item => Assert.NotNull(item));
==>
var item = Assert.Single(collection);
Assert.NotNull(item);

Assert.Collection(collection, item => { Assert.NotNull(item); });
==>
var item = Assert.Single(collection);
Assert.NotNull(item);

private void ElementInspector(object obj)
{ }

Assert.Collection(collection, ElementInspector);
==>
var item = Assert.Single(collection);
ElementInspector(item);

[bradwilson]

Thanks!