Given the following chained assertion

collection.Should().ContainSingle().Which.Parameters.Should().ContainSingle().Which.Should().Be(3)

If the second ContainSingle, fails, this PR ensures that the subject is displayed as

collection[0].Parameters[0].

This required several changes to make this possible:

• Establishes the term "subject identification" instead of "caller identifier"
• The existing CallerIdentifier has received a new method DetermineCallerIdentities that extracts all subjects from a chained assertion like the one above.
• The AssertionChain delegates tracking which subject to use for its failures using a new SubjectIdentificationBuilder class. This one also takes into account any ambient AssertionScopes and modifications that Which does to the subject/
• The names CallerIdentifier and CallerStatementBuilder are really not correct, but I've decided to keep that out of this PR.

Fixes #2747

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
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