Improved glossary/terminology at start - added unpointed and unboxed.
Created a section at start to describe the source language.  At the moment,
 all it contains is a description of unboxed tuple constructors.
Replaced erroneous uses of "closure" with "heap object".  According to
 the glossary, closures are enterable - things like stack objects
 are not enterable so they can't be closures.
Clarified section 2.7 (heap and stack checks): why should we not move Hp
 during heap check?
Added comment that I don't believe in the notion of fixed headers.