Divide chapter into sections.

Documentation/chap-garbage-collection.tex

@@ -1,11 +1,15 @@
 \chapter{Garbage collection}
 \label{chap-garbage-collection}
 
+\section{Introduction}
+
 Contrary to traditional operating systems such as \unix{}, a Lisp
 operating system will need a global \emph{garbage collector}.  There 
 is a rich literature (see e.g., \cite{Jones:2011:GCH:2025255}) on the
 subject. 
 
+\section{Per-thread garbage collector}
+
 Each thread has a local heap, roughly the size of the cache, say
 around 4MiB.  The thread-local heap is managed entirely by the thread
 itself, so that the garbage collector for it is executed by the thread
@@ -16,6 +20,8 @@ maintain allocation order.  This way, we have a precise measure of the
 relative age of the objects, so that we can promote only the oldest
 objects when required. 
 
+\section{Global garbage collector}
+
 In addition to the thread-local heaps, there is a global heap.  The
 garbage collector for this heap will use a combination of the
 traditional \emph{mark-and-sweep} collector and an ordinary memory