Provide interface for conversion binary data to formatted tuples

[olegrok]

All we know a problem - netbox:call converts tuples to simple Lua tables.
And it's OK. In general I don't know source space and structure of received tuples.
But if I receive data from the space I know it will be great to have an interface
to convert result of my call directly to tuples with format.

Currently it looks like

netbox = require('net.box')
netboxlib = require('net.box.lib')
buffer = require('buffer')
msgpackffi = require('msgpackffi')
msgpack = require('msgpack')
ffi = require('ffi')

-- Intro
box.cfg{listen = 3301}
box.schema.user.passwd('admin', 'test')

s = box.schema.space.create('test')
s:format({{name = 'id', type = 'string'}, {name = 'value', type = 'string'}})
s:create_index('pk')
s:replace({'key', 'value'})
c = netbox.connect('admin:test@localhost:3301')
function get()
    return s:select()
end

-- Byte juggling
call_buffer = buffer.ibuf()
c:call('get', {}, {buffer = call_buffer, skip_header = true})
-- After that I have a buffer
-- And I want to have an interface to directly deserialize this buffer
-- to tuples.
-- But I should wrap it with [IPROTO_DATA] header into map.
len, call_buffer.rpos = msgpack.decode_array_header(call_buffer.rpos, call_buffer:size())
buf = buffer.ibuf()
encode_fix = msgpackffi.internal.encode_fix
encode_r = msgpackffi.internal.encode_r
encode_fix(buf, 0x80, 1)
encode_r(buf, 0x30, 1)
wpos = buf:alloc(call_buffer:size())
ffi.copy(wpos, call_buffer.rpos, call_buffer:size())

tarantool> netboxlib.decode_select(buf.rpos, nil, c.space.test._format_cdata)
---
- - ['key', 'value']
- 'cdata<char *>: 0x010586803e'
...
tarantool> tuples = netboxlib.decode_select(buf.rpos, nil, c.space.test._format_cdata)
---
...

tarantool> tuples 
---
- - ['key', 'value']
...

tarantool> tuples[1]
---
- ['key', 'value']
...

tarantool> tuples[1]:tomap()
---
- 1: key
  2: value
  value: value
  id: key
...