fletcher 0.6.0

• Use binary_contains_exact if available from pyarrow instead of our own numba-based implementation.
• Provide two more consistent accessors:
• .fr_strx: Call efficient string functions on fletcher arrays, error if not available.
• .fr_str: Call string functions on fletcher and object-typed arrays, convert to object if no fletcher function is available.
• Add a numba-based implementation for strip, slice, and replace.
• Support LargeListArray as a backing structure for lists.
• Implement isnan ufunc.