Remove non_exhausive from Network

Issue rust-bitcoin#2225 is long and has many valid opposing opinions. The main
argument for having non_exhaustive is that it helps future proof the
ecosystem at the cost of pain now. The main argument against having
non_exhaustive is why have pain now when adding a network is so rare
that having pain then is ok.

At the end of the thread Andrew posts:

> I continue to think we should have an exhaustive enum, with a bunch of
> documentation about how to use it properly. I am warming up to the
> "don't have an enum, just have rules for defining your own" but I think
> this would be needless work for people who just want to grab an
> off-the-shelf set of networks or people who want to make their own enum
> but want to see an example of how to do it first.

In order to make some forward progress lets remove the `non_exhaustive`
now and backport this change to 0.32, 0.31, an 0.30.

Later we can add, and release in 0.33, whatever forward protection /
libapocalyse protection we want to add.

This removes the pain now and gives us a path to prevent future pain -
that should keep all parties happy.

Author: tcharding

bitcoin/src/network/mod.rs

@@ -59,9 +59,18 @@ pub enum TestnetVersion {
     V4,
 }
 
+
 /// The cryptocurrency network to act on.
+///
+/// This is an exhaustive enum, meaning that we cannot add any future networks without defining a
+/// new, incompatible version of this type. If you are using this type directly and wish to support the
+/// new network, this will be a breaking change to your APIs and likely require changes in your code.
+///
+/// If you are concerned about forward compatibility, consider using `T: Into<Params>` instead of this
+/// this type as a parameter to functions in your public API, or directly using the `Params` type.
+// For extensive discussion on the usage of `non_exhaustive` please see:
+// https://github.com/rust-bitcoin/rust-bitcoin/issues/2225
 #[derive(Copy, PartialEq, Eq, PartialOrd, Ord, Clone, Hash, Debug)]
-#[non_exhaustive]
 pub enum Network {
     /// Mainnet Bitcoin.
     Bitcoin,