Improve CRC-16 function

[felipediel]

Proposed changes

• Rename calculate_crc16 to crc16
• Apply PEP-8 naming conventions
• Remove unnecessary c_ushort type casting
• Remove unnecessary conversions
• Remove unnecessary bitwise operations
• Accept any sequence type
• Expose polynomial and initial value as kwargs
• Store the CRC-16 table for performance

Performance test

>>> import broadlink as blk
>>> from functools import partial
>>> from timeit import timeit
>>> b = bytearray(range(255))

# Before
>>> timeit(partial(blk.helpers.calculate_crc16, b))
826.9239850719969

# After
>>> timeit(partial(blk.helpers.crc16, b))
40.797150269994745