sql: support user-defined functions in SQL

Closes #2200
Closes #4113
Closes #2233

@TarantoolBot document
Title: The box.internal.sql_function_create is forbidden
Legacy mechanism box.internal.sql_function_create to make some
Lua function available in SQL is forbidden now.

To make some function available in SQL you need to use
box.schema.func.create() mechanism: you need to specify
1) function language and language-specific options(e.g. you
   are able to define a persistent Lua function)
2) whether this function is_deterministic or not: deterministic
   functions allows to generate more efficient SQL VDBE bytecode
   so you better specify it when it is true
3) the function returns type: a Tarantool type string describes
   a type of value returned by function
4) param_list - a table of Tarantool's types strings desccribe
   function argument types
5) exports - a table of Tarantool's frontends where this function
   should be available ('LUA' by default). You need to specify
   {'LUA', 'SQL'} to make function available both in SQL requests
   and visible in box.func folder

Example:
-- Case1: C function
-- function1.so has int divide() symbol
box.schema.func.create("function1.divide", {language = 'C',
			returns = 'number',
			param_list = {'number', 'number'},
			is_deterministic = true,
			exports = {'LUA', 'SQL'}})
box.execute('SELECT "function1.divide"(6, 3)')
- metadata:
  - name: '"function1.divide"(6, 3)'
    type: number
  rows:
  - [2]
box.schema.func.drop("function1.divide")

-- Case2: Persistent Lua function
box.schema.func.create("SUMMARIZE", {language = 'LUA',
			returns = 'number',
			body = 'function (a, b) return a + b end',
			param_list = {'number', 'number'},
			is_deterministic = true,
			exports = {'LUA', 'SQL'}})
box.execute('SELECT summarize(1, 2)')
- metadata:
  - name: summarize(1, 2)
    type: number
  rows:
  - [3]
box.schema.func.drop("summarize")

Moreover there is a special predefined Lua function LUA that
allows to evaluate a custom Lua expressions in SQL.
You need to pass a string in form "return ...." to LUA function
that returns more than one value of any type.

Example:
box.execute('SELECT lua(\'return 1 + 1\')')
- metadata:
  - name: lua('return 1 + 1')
    type: any
  rows:
  - [2]
box.execute('SELECT lua(\'return box.cfg.memtx_memory\')')
- metadata:
  - name: lua('return box.cfg.memtx_memory')
    type: any
  rows:
  - [268435456]