Add unsigned types to the system

[markehammons]

Unsigned types are important to C, and are frequently used. We can get away with a prototype version of Slinc not supporting them, but eventually we will need to have support for unsigned types in order to interact properly with C programs.

Numeric literals may be helpful for letting users express these types.

[markehammons]

Once unsigned types are developed, SizeT should be redefined as one.