avro-rs 0.13.0

# Migration Guide
## Unreleased
All changes are backward-compatible so far.

# 0.13.0
All changes are backward compatible.

# 0.12.0
All changes are backward compatible.

## 0.11.0
- A custom `Error` enum has been introduced to replace all existing errors and
  the `failure` crate has been replaced by `thiserror`.

  This means that all public functions returning `Result<T, failure::Error>`
  will now return `Result<T, avro::Error>` and that you can pattern match on
  `Error` variants if you want to gather more information about the error.

  For example, code that used to be like this:
  ```rust
  match decoded {
      Ok(msg) => Ok(msg.to_string()),
      // assuming you were reading a Duration
      Err(ref e) => match e.downcast_ref::<SchemaResolutionError>() {
          Some(_) => Ok("default".to_string()),
          None => Err(format!("Unexpected error: {}", e)),
      },
  }
  ```
  now becomes:
  ```rust
  match decoded {
      Ok(msg) => Ok(msg.to_string()),
      Err(Error::ReadDuration(_)) => Ok("default".to_string()),
      Err(e) => Err(format!("Unexpected error: {}", e)),
  }
  ```

  Please note that all instances of:
  - `DecodeError`
  - `ValidationError`
  - `DeError`
  - `SerError`
  - `ParseSchemaError`
  - `SchemaResolutionError`

  must be replaced by `Error`.

- The `ToAvro` trait has been deprecated in favor of `From<T>` for `Value` implementations.

  Code like the following:
  ```rust
  use crate::types::{Record, ToAvro, Value};

  let expected: Value = record.avro();
  ```

  should be updated to:

  ```rust
  use crate::types::{Record, Value};

  let expected: Value = record.into();
  ```

  Using the `ToAvro` trait will result in a deprecation warning. The trait will
  be removed in future versions.

- The `digest` crate has been updated to version `0.9`. If you were using the
  `digest::Digest` trait from version `0.8`, you must update to the one defined
  in `0.9`.

## 0.10.0
- `Writer::into_inner()` now calls `flush()` and returns a `Result`.

  This means that code like
  ```rust
  writer.append_ser(test)?;
  writer.flush()?;
  let input = writer.into_inner();
  ```

  can be simplified into
  ```rust
  writer.append_ser(test)?;
  let input = writer.into_inner()?;
  ```
  There is no harm in leaving old calls to `flush()` around.