Response to usage of unwrap in non-test code

This commit replaces all usage of `unwrap` with `expect` within
`networks/monero`, clarifying why the panic risked is unreachable. This commit
also replaces some uses of `unwrap` with solutions which are guaranteed not to
fail.

Notably, compilation on 128-bit systems is prevented, ensuring
`u64::try_from(usize::MAX)` will never panic at runtime.

Slight breaking changes are additionally included as necessary to massage out
some avoidable panics.

networks/monero/primitives/src/lib.rs

@@ -124,7 +124,7 @@ impl Commitment {
   /// defined serialization.
   pub fn serialize(&self) -> Vec<u8> {
     let mut res = Vec::with_capacity(32 + 8);
-    self.write(&mut res).unwrap();
+    self.write(&mut res).expect("write failed but <Vec as io::Write> doesn't fail");
     res
   }
 
@@ -230,7 +230,7 @@ impl Decoys {
   pub fn serialize(&self) -> Vec<u8> {
     let mut res =
       Vec::with_capacity((1 + (2 * self.offsets.len())) + 1 + 1 + (self.ring.len() * 64));
-    self.write(&mut res).unwrap();
+    self.write(&mut res).expect("write failed but <Vec as io::Write> doesn't fail");
     res
   }