Add util to get the size of Error and such.

Author: Yamakaky

examples/size.rs

@@ -0,0 +1,30 @@
+#[macro_use]
+extern crate error_chain;
+
+use std::mem::{size_of, size_of_val};
+
+error_chain! {
+    errors {
+        AVariant
+        Another
+    }
+}
+
+fn main() {
+    println!("Memory usage in bytes");
+    println!("---------------------");
+    println!("Result<()>: {}", size_of::<Result<()>>());
+    println!("  (): {}", size_of::<()>());
+    println!("  Error: {}", size_of::<Error>());
+    println!("    ErrorKind: {}", size_of::<ErrorKind>());
+    let msg = ErrorKind::Msg("test".into());
+    println!("      ErrorKind::Msg: {}", size_of_val(&msg));
+    println!("        String: {}", size_of::<String>());
+    println!("    State: {}", size_of::<error_chain::State>());
+    let state = error_chain::State {
+        next_error: None,
+        backtrace: None,
+    };
+    println!("      State.next_error: {}", size_of_val(&state.next_error));
+    println!("      State.backtrace: {}", size_of_val(&state.backtrace));
+}