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'.

Documentation/t/testitem.md

@@ -1,87 +0,0 @@
-% redo(1) Redo %VERSION%
-% Avery Pennarun <apenwarr@gmail.com>
-% %DATE%
-
-# NAME
-
-redo-always - mark the current target as always needing to be rebuilt
-
-# SYNOPSIS
-
-redo-always
-
-
-# DESCRIPTION
-
-Normally redo-always is run from a .do file that has been
-executed by `redo`(1).  See `redo`(1) for more details.
-
-This is a "quoted string."
-
-"entirely quoted"
-
-.starting with dot
-
-I'm a \big \\backslasher!
-
-This **line** has _multiple_ `formats`(interspersed) *with* each _other_
-and *here is a multi
-line italic.*
-
-This line has an & and a < and a >.
-
-- "line starting with quoted"
-
-        Here's some code
-           with indentation
-              yay! (a \backslash and <brackets>)
-              "foo"
-        
-        skipped a line
-           indented
-           
-              another skip
-              
-- -starting with dash
-
-- .starting with dot
-
-chicken
-
--   list item
-    with more text
-  
-    and even more
-    
-    -   second list
-     	-   third list
-
-wicken
-
--   list 1a
--   list 1b
-    -   list 2
-        -   list 3
-
-barf
-          
-First line
-:    definition list.
-     with
-     multiple
-     lines!
-     
---item=*value*
-:    a description.
-
-`-x`
-:    more stuff.  if you had a lot of text, this is what it
-would look like.  It goes on and on and on.
-
-a line with *altogether* "too much" stuff on it to realistically *make* it in a definition list
-:   and yet, here we are.
-
-
-# SEE ALSO
-
-`redo`(1), `redo-ifcreate`(1), `redo-ifchange`(1), `redo-stamp`(1)