Switch from failure to anyhow

[alexcrichton]

This moves us back to std::error::Error compatibility and helps build
times since it doesn't rely on an older version of syn

[fitzgen]

nice, living in the future

[fitzgen]

https://travis-ci.org/rustwasm/walrus/jobs/596675888#L861-L865

error[E0599]: no method named `iter_chain` found for type `anyhow::error::Error` in the current scope
 --> examples/round-trip.rs:8:20
  |
8 |         for c in e.iter_chain() {
  |                    ^^^^^^^^^^
error: aborting due to previous error

Need to update the examples too.