For pure TLV messages such as Gossip 1.75 and Bolt12 messages, there are
defined TLV ranges for unsigned data vs signed data. The signed ranges are
the inclusive ranges of 0 to 159 and 1000000000 to 2999999999.
What we do here in this commit is: 1) update the number of main types
generated so that they include type 160 which is the first type in the
un-signed range and we will make use of this type for signature fields
in gossip messages. Next, we add two marker types so that we can define
the rest of the ranges.
With this change, callers can now examine a struct instance of the
`TlvType` instance to figure out programmatically which integer type is
maps to. This'll be useful when decoding optional TLV types into a wire
struct (knowing when to set it to Some vs None).
In this commit, we add some new code generation to the codebase. As
we'll see in a future commit, this'll allow us to create a new Record[T,
V] type, where T is actually a concrete _struct_ that implements a
special interface that deems it as a valid TLV type.
