digest: introduce FFI bindings for xxHash32/64

This patch introduces new hash types for digest module - xxHash32
and xxHash64.

Closes #2003

@TarantoolBot document
Title: digest module supports xxHash32/64

```lua
-- Examples below demonstrate xxHash32.
-- xxHash64 has exactly the same interface

-- Calculate the 32-bits hash (default seed is 0).
digest.xxhash32(string[, seed])

-- Streaming
-- Start a new hash by initializing state with a seed.
-- If no value provided, 0 is used as default.
xxhash = digest.xxhash32.new([seed])
-- Also it's possible to specify seed manually. If no value
-- provided a value initially passed to "new" is used.
-- Here and below "seed" expected to be unsigned
-- number. Function returns nothing.
xxhash:clear([seed])
-- Feed the hash state by calling "update" as many times as
-- necessary. Function returns nothing.
xxhash:update('string')
-- Produce a hash value.
xxhash:result()
```