Reads data directly from memory.
- type| can be either 'BYTE or 'WORD.
- address| is a 20-bit address
(for instance #xB8000 is address of the main text
screen buffer for color adapters)
- length| is optional. If not given, PEEK returns a single number.
Otherwise, PEEK| returns a list of contents of consecutive memory
locations, starting with address and incrementing with size of type|.