Closes #1260

@TarantoolBot document
Title: introduce func indexes in memtx
Now you can define a func_index using a registered persistent
function.

There are restrictions for function and key definition for
a functional index:
 - the referenced function must be persistent, deterministic
   and must return a scalar type or an array.
 - you must define key parts which describe the function return value
 - the function must return data which types match the
   defined key parts
 - the function may return multiple keys; this would be a multikey
   functional index; each key entry is indexed separately;
 - for multikey functional indexes, the key definition should
   start with part 1 and cover all returned key parts
 - key parts can't use JSON paths.
 - the function used for the functional index can not access tuple
   fields by name, only by index.

Functional index can't be primary.
It is not possible to change the used function after a functional
index is defined on it. The index must be dropped first.

Each key returned by functional index function (even when it is a
single scalar) must be returned as a table i.e. {1} and must
match the key definition.

To define a multikey functional index, create a function with
opts = {is_multikey = true} and return a table of keys.

Example:
s = box.schema.space.create('withdata')
s:format({{name = 'name', type = 'string'},
          {name = 'address', type = 'string'}})
pk = s:create_index('name', {parts = {1, 'string'}})
lua_code = [[function(tuple)
                local address = string.split(tuple[2])
                local ret = {}
                for _, v in pairs(address) do
			table.insert(ret, {utf8.upper(v)})
		end
                return ret
             end]]
box.schema.func.create('address', {body = lua_code,
                       is_deterministic = true, is_sandboxed = true,
                       opts = {is_multikey = true}})
idx = s:create_index('addr', {unique = false,
                     func = 'address',
                     parts = {{1, 'string', collation = 'unicode_ci'}}})
s:insert({"James", "SIS Building Lambeth London UK"})
s:insert({"Sherlock", "221B Baker St Marylebone London NW1 6XE UK"})
idx:select('Uk')
---
- - ['James', 'SIS Building Lambeth London UK']
  - ['Sherlock', '221B Baker St Marylebone London NW1 6XE UK']
...