Implement half-width katakana support

[exoego]

Closes #19

• Adds utils/halfwidth_katakana_to_hiragana.
  • Counterintuitively, this is not "half to full" conversion. Because, ..._to_hiragana util can be leveraged in to_hiragana's character loops so the input is not enumerated multiple times.
• In to_katakana and to_romaji, the util is invoked only when input contains half-width kana,
  • The input may be fully enumerated at worst case when no half-width katakana is used.
• to_haflwidth_katakana or new option is NOT added, since I don't need such.