Make MapFieldNames public (#2118) (#2134)

Author: tustvold

arrow/src/array/builder/mod.rs

@@ -52,7 +52,7 @@ pub use fixed_size_list_builder::FixedSizeListBuilder;
 pub use generic_binary_builder::GenericBinaryBuilder;
 pub use generic_list_builder::GenericListBuilder;
 pub use generic_string_builder::GenericStringBuilder;
-pub use map_builder::MapBuilder;
+pub use map_builder::{MapBuilder, MapFieldNames};
 pub use primitive_builder::PrimitiveBuilder;
 pub use primitive_dictionary_builder::PrimitiveDictionaryBuilder;
 pub use string_dictionary_builder::StringDictionaryBuilder;