Decide: rename Size subclasses to indicate unsigned and bit counts

I wanted to make sure this got specifically addressed before a stem.client release, since after that point it might be too late (due to backwards-compatibility concerns).

From [[ticket:26227#comment:4]]:

==== Naming: stem.client.datatype.Size subclasses/attributes Suggestion: It might be good to switch CHAR/SHORT/etc. to UCHAR/USHORT/etc. I don't know what the convention is here, but it may help for readability.

I'm personally used to U<size> to signify unsigned and <size> to signify signed. I think switching to U<size> would make the code //potentially// easier to read for newcomers from various backgrounds.

Suggestion: Similarly, it may help to put the bits length in it, too - for the most immediate readability. So, e.g.:

• UCHAR8
• USHORT16
• etc.

For reference, on a quick glance...

• [[https://gitweb.torproject.org/trunnel.git/tree/README|trunnel]] [to use] u8, u16, etc.
• tor [to use] uint8_t, uint16_t, etc.

atagar: thoughts?