Expose packager and transporter structs for use with NewClient2

[andig]

Fix https://github.com/grid-x/modbus/issues/56

This PR exposes packers and transporters. It does not modify existing api:

• make packager/transporter fields public members of client handlers
• add New*Transporter methods

Notes on implementation:

• Use of public fields in the exposed client requires exposing the package/transporter types. With private members, the types returned by the New*Transporter methods would be "annoying" for the linter.
• There is one open inconsistency with the current approach: while serial transports hide the serial port as implementation detail, the *TCP transports expose the underlying TCP transport. That can be changed, if desired consistently for both cases.
• exposing TCPTransporter (which is an implementation detail) as part of the ASCII/RTUOverTCP transporters is unfortunate but required to keep the public fields visible.
• serial transporters Printf() method was removed- it seems unused (will split into separete PR)

TODO

• [ ] pointer receiver for serial transport is necessary to avoid copying of lock in serial port
• [ ] consistently make all transports pointer receivers
• [ ] decide if same is desired for packagers
• [ ] decide future of rtu/ascii Printf() public api. With making serialTransport public, the transport's logger should be used instead. Proposal: remove Printf() (potential though minor BC break).

Alternatives:

Add a Clone() method that clones the entire client handler, sharing the underlying transport https://github.com/grid-x/modbus/pull/70.

Maybe think about something like:

// WithPackager returns a new TCPClientHandler with given packager.
func (h *TCPClientHandler) WithPackager(p TCPPackager) {
    h.TCPPackager = p
}