ARROW-1246: [Format] Draft Flatbuffer metadata description for Map

Author: Wes McKinney <[email protected]>

Closes apache#876 from wesm/ARROW-1246 and squashes the following commits:

98790dfc [Wes McKinney] Review feedback to clarify nullability of map components
346b48dd [Wes McKinney] Typo
06ae8ebf [Wes McKinney] Draft Flatbuffer metadata for Map

format/Schema.fbs

@@ -44,6 +44,35 @@ table FixedSizeList {
   listSize: int;
 }
 
+/// A Map is a logical nested type that is represented as
+///
+/// List<entry: Struct<key: K, value: V>>
+///
+/// In this layout, the keys and values are each respectively contiguous. We do
+/// not constrain the key and value types, so the application is responsible
+/// for ensuring that the keys are hashable and unique. Whether the keys are sorted
+/// may be set in the metadata for this field
+///
+/// In a Field with Map type, the Field has a child Struct field, which then
+/// has two children: key type and the second the value type. The names of the
+/// child fields may be respectively "entry", "key", and "value", but this is
+/// not enforced
+///
+/// Map
+///   - child[0] entry: Struct
+///     - child[0] key: K
+///     - child[1] value: V
+///
+/// Neither the "entry" field nor the "key" field may be nullable.
+///
+/// The metadata is structured so that Arrow systems without special handling
+/// for Map can make Map an alias for List. The "layout" attribute for the Map
+/// field must have the same contents as a List.
+table Map {
+  /// Set to true if the keys within each value are sorted
+  keysSorted: bool;
+}
+
 enum UnionMode:short { Sparse, Dense }
 
 /// A union is a complex type with children in Field
@@ -170,7 +199,8 @@ union Type {
   Struct_,
   Union,
   FixedSizeBinary,
-  FixedSizeList
+  FixedSizeList,
+  Map
 }
 
 /// ----------------------------------------------------------------------