Make examples self-sufficient as far as possible

[DifferentialOrange]

Since all our examples require a Tarantool instance to connect, it wouldn't be possible to make a fully executable example. (Maybe we should check other database connnectors and see if they have found some way.) But at least we should make sure every other thing is ok.

When I run CustomUnpacking example on https://pkg.go.dev/github.com/tarantool/go-tarantool with Run

it fails with

go: finding module for package github.com/tarantool/go-tarantool
go: finding module for package gopkg.in/vmihailenco/msgpack.v2
go: downloading gopkg.in/vmihailenco/msgpack.v2 v2.9.2
go: downloading github.com/tarantool/go-tarantool v0.0.0-20220420113901-1d06618833d3
./prog.go:14:12: undefined: Member
./prog.go:23:12: undefined: Member
./prog.go:58:21: undefined: Member
./prog.go:93:53: undefined: Member