PARQUET-758: Add Float16/Half-float logical type (#184)

Type involves a trade-off of reduced precision,
in exchange for more efficient storage.

LogicalTypes.md

@@ -245,6 +245,16 @@ comparison.
 To support compatibility with older readers, implementations of parquet-format should
 write `DecimalType` precision and scale into the corresponding SchemaElement field in metadata.
 
+### FLOAT16
+
+The `FLOAT16` annotation represents half-precision floating-point numbers in the 2-byte IEEE little-endian format.
+
+Used in contexts where precision is traded off for smaller footprint and potentially better performance.
+
+The primitive type is a 2-byte fixed length binary.
+
+The sort order for `FLOAT16` is signed (with special handling of NANs and signed zeros); it uses the same [logic](https://github.com/apache/parquet-format#sort-order) as `FLOAT` and `DOUBLE`.
+
 ## Temporal Types
 
 ### DATE

src/main/thrift/parquet.thrift

@@ -242,6 +242,7 @@ struct MapType {}     // see LogicalTypes.md
 struct ListType {}    // see LogicalTypes.md
 struct EnumType {}    // allowed for BINARY, must be encoded with UTF-8
 struct DateType {}    // allowed for INT32
+struct Float16Type {} // allowed for FIXED[2], must encoded raw FLOAT16 bytes
 
 /**
  * Logical type to annotate a column that is always null.
@@ -355,6 +356,7 @@ union LogicalType {
   12: JsonType JSON           // use ConvertedType JSON
   13: BsonType BSON           // use ConvertedType BSON
   14: UUIDType UUID           // no compatible ConvertedType
+  15: Float16Type FLOAT16     // no compatible ConvertedType
 }
 
 /**