Rename SerializerProvider as SerializationContext (for 3.0)

[cowtowncoder]

Something that should have been done during 1.x->2.0 major version upgrade but better late than never: due to historical reasons (serialization implemented before deserialization), context object naming is inconsistent. We have:

• DeseralizationContext for deserialization
• SerializerProvider for serialization

Latter may have made sense in 0.9.4 since the primary reason was access to child serializers (and this is still an importan use case); but conceptually it is a context object, and has expanded a lot in functionality.
So let's fix this naming discrepancy in 3.0.
(yes, this will result in lots of changes across the whole codebase)