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lua: introduce utf8 built-in globaly visible module

utf8 is a module partially compatible with Lua 5.3 utf8 and
lua-utf8 third party module.
Partially means, that not all functions are implemented.

The patch introduces these ones:
upper, lower, len, char, sub, next.

Len and char works exactly like in Lua 5.3. Other functions work
like in lua-utf8.

Tarantool utf8 has extensions:

* isupper/lower/alpha/digit, that check some property by a symbol
  or by its code;

* cmp/casecmp, that compare two UTF8 strings.

Closes #3290
Closes #3385
Closes #3081

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