add support for partial deserialization of Thrift

I work for Pinterest. I developed a technique for partial deserialization of Thrift that has been very useful in significantly improving efficiency of the data processing at Pinterest. I would like to contribute that feature to Apache Thrift. More details on this technique are available in this blog I recently wrote:
https://medium.com/pinterest-engineering/improving-data-processing-efficiency-using-partial-deserialization-of-thrift-16bc3a4a38b4