Works by maintaining the xxHash32 state variables (v1 -> v4, length) as well as a queue of values that fall outside of the block size (16 bytes/4 ints). The seed is initialized to random bytes.

Further details, unit tests and history: dotnet/corefx#25013

* Removing AgressiveInlining from all public facing methods.
* Fixing erroneous comment

* Adding comment explaining why Combine(value1) performs a hash.
* Removing superfluous code and attributes.
* Improving perf of Add(..., null).
* Moving two lines of code below the giant comment to improve reading locality.
* Use mod instead of bit twiddling.

* Improving some comments. Ensuring comments wrap at column 80.
* Fixing GenerateGlobalSeed().
* Reading _length once in methods.
* Ignoring overflow.
* Simplifying some conditions.
* Using goto for fallthrough in ToHashCode().
* Adding Equals as a disallowed method.
* Adding exception message string.

* Adding further detail to obsolete and exception message.
 * Fix spacing issues.
* Restore nested if for ToHashCode.

* Correcting goto comment.
* Using different strings for errors.
* Re-ordering a comment so that it makes more sense.
* Fixing comment that was still difficult to understand.

* Removing AggressiveInlining on big method.
* Reducing number of generic instantiations for Add<T>.

* Comment.

Slightly optimize Add<T>(,).