Patch by Reid Kleckner!

lnotab-based tracing is very complicated and isn't documented very well.  There
were at least 3 comment blocks purporting to document co_lnotab, and none did a
very good job. This patch unifies them into Objects/lnotab_notes.txt which
tries to completely capture the current state of affairs.

I also discovered that we've attached 2 layers of patches to the basic tracing
scheme. The first layer avoids jumping to instructions that don't start a line,
to avoid problems in if statements and while loops.  The second layer
discovered that jumps backward do need to trace at instructions that don't
start a line, so it added extra lnotab entries for 'while' and 'for' loops, and
added a special case for backward jumps within the same line. I replaced these
patches by just treating forward and backward jumps differently.

Most uses of PyCode_Addr2Line
(http://www.google.com/codesearch?q=PyCode_Addr2Line) are just trying to get
the line number of a specified frame, but there's no way to do that directly.
Forcing people to go through the code object makes them know more about the
guts of the interpreter than they should need.

The remaining uses of PyCode_Addr2Line seem to be getting the line from a
traceback (for example,
http://www.google.com/codesearch/p?hl=en#u_9_nDrchrw/pygame-1.7.1release/src/base.c&q=PyCode_Addr2Line),
which is replaced by the tb_lineno field.  So we may be able to deprecate
PyCode_Addr2Line entirely for external use.

Most uses of PyCode_New found by http://www.google.com/codesearch?q=PyCode_New
are trying to build an empty code object, usually to put it in a dummy frame
object. This patch adds a PyCode_NewEmpty wrapper which lets the user specify
just the filename, function name, and first line number, instead of also
requiring lots of code internals.

The new PyParser_*Ex() functions are based on Neal's suggestion and initial patch. The new __future__ feature makes all '' and r'' unicode strings. b'' and br'' stay (byte) strings.

This work is substantially Anthony Baxter's, from issue
1633807.  I just freshened it, made a few minor tweaks,
and added the test cases.  I also created issue 2412,
which is to check for 2to3's behavior with the print
function.  I also added myself to ACKS.

backwards compatibility. Add Py_Refcnt, Py_Type, Py_Size, and
PyVarObject_HEAD_INIT.