• Discover characteristic descriptors during service discovery
• Allow specifying Characteristic or Descriptor data length, and allow setting to fixed length.
• Add GATTC/GATTS read/write for Descriptor.
• Remove named constants for descriptor UUIDs. Better to just do these in Python.
• change Characteristic properties from keyword arguments to bitmask.
• Add setting read/write security levels on Characteristic and Descriptor. Introduce Attribute class as a notional superclass of Characteristic and Descriptorwhich cannot be instantiated: it just provides security constants now.
• Fix Adapter.address fetch.
• Make default name CIRCUITPY be more unique by suffixing last four hex digits of address, e.g. CIRCUITPYa1c4.
• Add peripheral client pairing (no bonding yet).
• Allow Peripheral to discover services provided by a central.

These changes and additions sync up with adafruit/Adafruit_CircuitPython_BLE#16.

Non-BLE:

• Delete obsolete ports/nrf/make-pins.py.
• Fix badly formatted time_struct documentation.
• Add nibble_to_hex_upper[16] and nibble_to_hex_lower[16] arrays for easy printing of hex digits.