Move main.rs to a test, drop dep on png crate

The codec library doesn't use any external dependencies. The PNG crate
is only to load some source material for testing the codec. Turning the
test driver into a real test lets me turn the PNG dependency into a
dev-dependency.

Cargo.toml

@@ -5,5 +5,5 @@ edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
-[dependencies]
+[dev-dependencies]
 png = "0.17.14"