arrow

ARROW-4141: [Ruby] Add support for creating schema from raw Ruby objects

The followings should be implemented by follow-up works:

  * Arrow::TimestampDataType.new(unit: ...)
  * Arrow::Time32DataType.new(unit: ...)
  * Arrow::Time64DataType.new(unit: ...)
  * Arrow::DecimalDataType.new(precision: ..., scale: ...)
  * Arrow::SparseUnionDataType.new(fields: ..., type_codes: ...)
  * Arrow::DenseUnionDataType.new(fields: ..., type_codes: ...)
  * Arrow::DictionaryDataType.new(fields: ..., type_codes: ...)

Author: Kouhei Sutou <kou@clear-code.com>

Closes #3293 from kou/ruby-schema-new and squashes the following commits:

d251ba9d <Kouhei Sutou> Add .yardopts to rat exclude files
169b8656 <Kouhei Sutou>  Add support for creating schema from raw Ruby objects