Easier specification of prefixes; fixes for use with bleak

[dhalbert]

• Advertisement-matching prefixes are now specified as tuples of prefixes. You no longer need to count up the bytes and include a length byte in front of each subprefix. The class attribute has been renamed from prefix to match_prefixes. This is an incompatible change; a few other libraries will need to change also. PR's will be opened for those libraries.
• All the definitions of __get__(self, obj, cls) now check for obj is None, and return self in that case. This is because MicroPython/CircuitPython don't invoke descriptor protocol on obj's class, but CPython does. So to be uniform, when running in CPython, check for this case and just return the actual object.
• Catch when microcontroller is not present, such as when running in CPython.
• Windows doesn't have os.uname(), so catch that.
• Make it easier and clearer for an Advertisement subclass to change the behavior of matches() from all to any, by providing a match_prefixes(cls, entry, all_=True) class method to call. Note that the arg is called all_, because it otherwise conflicts with the builtin function all(). See _bleio.ScanEntry.matches(..., all=) keyword arg conflicts with all() circuitpython#3007 for a related issue.
• Improve printed representation of some classes.

Fixes #82.

[tannewt]

Just a couple comments. Thanks!

[tannewt]

Ok, thanks!