Added initial implementation and sanity tests for AvroFixed, accompanied by utility classes FixedData and FixedDataCompanion.
Changed the length enforcement strategy for FixedData elements (now uses static annotations instead of more complex companion object logic).  Made attendant changes to AvroType.fromType, added tests in the util and core packages.
Updated the definition of c.g.scalavro.util.FixedData to take an explicitly immutable Seq[Byte] as its constructor argument.  Added simple tests to AvroTypeSpec in core.
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.
Added a condition in AvroType.fromType for subclasses of FixedData to ensure that the supplied type does not take type parameters, as instance creation would fail upon subsquent calls to AvroFixedIO.read
Added a test type and a test for the case where a FixedData subclass is a parameterized type.  Such classes should cause calls to AvroType.fromType to fail with an IllegalArgumentException.