EXECUTING A WORD

At it's core, executing a word is pushing the wordref on PS and
calling EXECUTE. Then, we let the word do its things. Some
words are special, but most of them are of the compiledWord
type, and that's their execution that we describe here.

First of all, at all time during execution, the Interpreter
Pointer (IP) points to the wordref we're executing next.

When we execute a compiledWord, the first thing we do is push
IP to the Return Stack (RS). Therefore, RS' top of stack will
contain a wordref to execute next, after we EXIT.

At the end of every compiledWord is an EXIT. This pops RS, sets
IP to it, and continues.