Restructured library documentation

Doc/lib/libintro.tex

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+\chapter{Introduction}
+
+The Python library consists of three parts, with different levels of
+integration with the interpreter.
+Closest to the interpreter are built-in types, exceptions and functions.
+Next are built-in modules, which are written in \C{} and linked statically
+with the interpreter.
+Finally there are standard modules that are implemented entirely in
+Python, but are always available.
+For efficiency, some standard modules may become built-in modules in
+future versions of the interpreter.
+\indexii{built-in}{types}
+\indexii{built-in}{exceptions}
+\indexii{built-in}{functions}
+\indexii{built-in}{modules}
+\indexii{standard}{modules}
+\indexii{\C{}}{language}