Directory reorg: move code into redo/, generate binaries in bin/.

It's time to start preparing for a version of redo that doesn't work
unless we build it first (because it will rely on C modules, and
eventually be rewritten in C altogether).

To get rolling, remove the old-style symlinks to the main programs, and
rename those programs from redo-*.py to redo/cmd_*.py.  We'll also move
all library functions into the redo/ dir, which is a more python-style
naming convention.

Previously, install.do was generating wrappers for installing in
/usr/bin, which extend sys.path and then import+run the right file.
This made "installed" redo work quite differently from running redo
inside its source tree.  Instead, let's always generate the wrappers in
bin/, and not make anything executable except those wrappers.

Since we're generating wrappers anyway, let's actually auto-detect the
right version of python for the running system; distros can't seem to
agree on what to call their python2 binaries (sigh). We'll fill in the
right #! shebang lines.  Since we're doing that, we can stop using
/usr/bin/env, which will a) make things slightly faster, and b) let us
use "python -S", which tells python not to load a bunch of extra crap
we're not using, thus improving startup times.

Annoyingly, we now have to build redo using minimal/do, then run the
tests using bin/redo.  To make this less annoying, we add a toplevel
./do script that knows the right steps, and a Makefile (whee!) for
people who are used to typing 'make' and 'make test' and 'make clean'.

docs/cookbook/defaults/default.do

@@ -0,0 +1,25 @@
+# $1 is the target name, eg. test.txt
+# $2 in the same as $1.  We'll talk about
+#    that in a later example.
+# $3 is the temporary output file we should
+#    create.  If this script is successful,
+#    redo will atomically replace $1 with $3.
+
+if [ -e "$1.in" ]; then
+    # if a .in file exists, then do some
+    # text substitution.
+    #
+    # Remember, the user asks redo to build
+    # a particular *target* name.  It's the .do
+    # file's job to figure out what source file(s)
+    # to use to generate the target.
+    redo-ifchange "$1.in" version date
+    read VERSION <version
+    read DATE <date
+    sed -e "s/%VERSION%/$VERSION/g" \
+        -e "s/%DATE%/$DATE/g" \
+        <$1.in >$3
+else
+    echo "$0: Fatal: don't know how to build '$1'" >&2
+    exit 99
+fi