Adding xxHash64 and xxHash32 functions

[filimonov]

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For changelog. Remove if this is non-significant change.

Category (leave one):

• New Feature

Short description (up to few sentences):
xxHash64 and xxHash32 hashing functions added.

Detailed description (optional):
xxHash functions are fast (~as fast as metroHash), widely used in different usecases, and well portable. It's quite popular - even in ClickHouse contrib it exists 4 times:

./contrib/llvm/llvm/include/llvm/Support/xxhash.h
./contrib/librdkafka/src/xxhash.h
./contrib/lz4/lib/xxhash.h
./contrib/zstd/contrib/linux-kernel/include/linux/xxhash.h
./contrib/zstd/lib/common/xxhash.h

Added to simplify integrations.

I didn't add it as a submodule, used implementation from lz4 which is already included. (Motivation: no extra dep, lz4 - is core ClickHouse part, and author(xxHash) in authors(lz4))

[alexey-milovidov]

Ok. And what about performance test?