to_str-1.0: Improve Error to_string

Related to #79
to_str-1.0: Improve Hello to_string

A Hello message may contain an uninterpreted body. Provide some
indication if it does or not.

Related to #79
to_str-1.0: Improve EchoRequest/Reply to_string

These messages may contain an uninterpreted body, so provide some
indication of whether or not they do.

Related to #79.
to_str-1.0: Improve top-level Message to_string

Many of the messages had to_string functions that simply weren't being
called, one exception being VendorMsg.

Related to #79.
to_str-1.0: Replace ConfigReply with SwitchConfig

The two are redundant.

Related to #79.
to_str-1.0: Make constants consistent with spec

... when possible.

Related to #79.
Merge pull request #106 from frenetic-lang/to_str-1.0

Improve Message.to_string for OpenFlow0x01