[Avro] Add support for @Stringable annotation

[baharclerode]

This adds support for the @Stringable annotation, and adds support for stringable classes (Those with @Stringable, and URL/URI/File/BigInteger/BigDecimal). This fixes BigInteger/BigDecimal compatibility with the apache implementation.

Of particular note, BigDecimal is no longer a Type.DOUBLE in schema generation, nor is BigInteger a Type.LONG in schema generation (since they are "stringable" to avro), but they should continue to serialize/deserialize properly from int/long/float/double scalars in existing avro schemas.

I treated @Stringable on a class as having a creator annotation on the constructor which takes a single string argument. I'm not sure if there is a better way to do this.

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Rebased onto master as per this comment.

[cowtowncoder]

Hmmh. This change seems to have nasty performance downside: serialization 30% slower, deserialization 5-15%...

[cowtowncoder]

Ok, was able to make minor changes to get serialization perf back: I do not think @Stringable check ought to be needed on actual write path (since annotation lookups are remarkable ineffecient). May do more tweaks, but works fine for now.