Remove Response type from public API

[FZambia]

Related #7

Removing Response type from public API. This should make error handling more straightforward. I.e. eliminate code like this:

resp, err = conn.Exec(...)
if err != nil {
    // Handle error
}
if resp.Error != "" {
  // Handle Response.Error
}
// Handle Response.Data

While it was not really necessary having Error in Response struct seems frustrating.

The code above becomes:

result, err = conn.Exec(...)
if err != nil {
    // Handle error
}
// Handle result which is []interface{} – i.e. what was previously in Response.Data

Changes:

gorelease -base v0.2.3 -version v0.3.0
# github.com/FZambia/tarantool
## incompatible changes
(*Connection).Exec: changed from func(*Request) (*Response, error) to func(*Request) ([]interface{}, error)
(*Connection).ExecContext: changed from func(context.Context, *Request) (*Response, error) to func(context.Context, *Request) ([]interface{}, error)
(*Request).WithPush: changed from func(func(*Response)) *Request to func(func([]interface{})) *Request
ErrorCodeBit: removed
Future.Get: changed from func() (*Response, error) to func() ([]interface{}, error)
FutureContext.GetContext: changed from func(context.Context) (*Response, error) to func(context.Context) ([]interface{}, error)
OkCode: removed
Response: removed

# summary
v0.3.0 is a valid semantic version for this release.