StructBuilder

[pelletier]

Fixes #8

The goal is to be able to read rows from Parquet files into Go values.

In addition, we eventually would like the following properties:

• Easy-to-use API to read rows.
• Predicate push-down (only read the minimum values to decide whether to read a whole row) RowReader predicates #10
• Projection (only read the minimal amount of columns) RowReader projection #9
• Schema stays a representation of a Parquet schema (to reuse in the writer).

BenchmarkReflectReadRow
BenchmarkReflectReadRow-8            	      38	  32238412 ns/op	20581119 B/op	  338369 allocs/op
BenchmarkReflectReadRowParquetGo
BenchmarkReflectReadRowParquetGo-8   	       5	 220576040 ns/op	93017072 B/op	 1473595 allocs/op