Add missing array_api functions

[guilhermeleobas]

• [x] std
• [x] var
• [x] arange
• [x] asarray
• [x] argmax
• [x] argmin
• [x] nonzero
• [x] unique_values
• [x] Array.drop_null
• [x] astype
• [x] iinfo
• [x] finfo
• [x] result_type
• [x] can_cast