Added a ReflectionHelpers.singleArgumentConstructor method in the util sub-project, which optionally returns a method mirror for the public single-arg constructor method given type tags for the class and formal parameter types.
Added a condition to AvroType.fromType[T] to ensure that subclasses of FixedData have a public single-argument constructor taking an immutable.Seq[Byte].
Modified the definition of c.g.scalavro.util.FixedData to specify the `bytes` constructor parameter to be explicitly an immutable sequence.  Also modified this member to be a public val (which subclasses must override).  Changed use sites in various test fixtures to conform this this changed API.
Implemented AvroFixedIO.read and AvroFixedIO.write, added some simple tests.
Modified the implementation of AvroBytesIO.read to match that of AvroFixedIO.read.  The previous implementation may have read the wrong number of bytes from the supplied input stream.