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). Seed is assumed to be zero as there is no seed ctor in the proposal - however, it is possible to add a seed without changing the structure.

Tests against known xxHash32 vectors are provided ("abcd1234efgh5678...").

Order of members now matches the proposal, if that matters. Combine methods are unrolled for perf. Removed superfluous attributes.

Test vector tests are now optional (behind SYSTEM_HASHCODE_TESTVECTORS def) as the random seed will break these tests. Defining SYSTEM_HASHCODE_TESTVECTORS will disable the per-AppDomain random seed. Combine is now tested against the behavior of Add.

PR Feeback

* Code style cleanups.
* Unessecary fields and structlayout removed from ref.
* Correct attribution applied.
* More perf tests.
* Perf tests prevent the jitter from removing dead code that we might care about.
* Fixed false positive in tests.
* Don't use float types for tests.
* Remove AllowUnsafeBlocks.
* Disable compilation for netfx.
* Removing 0 prefix from constants.
* Removing unchecked scopes.
* Removing 4 reads of s_seed in Initialize.
* Ensure that all methods are inlined. Some parameters changed to ref to do this.
* Changing exception to NotSupportedException.
* GetHashCode is NoInlining so that it is display in the stack trace.

* Fix test issue.
* Remove unnecessary optimization.
* Default comparer to EqualityComparer<T>.Default.
* Add info on how test vectors are added.

* Remove HashCode.cs