Add a ".bash" extension to the main include script make explicit the
intent for it to be used with Bash.

Split the bindings from the function definitions. This will allow users
to source the functions and define their own bindings. The bindings file
can be sourced to get the default bindings.

I had used `C-x +` and `C-x -` during testing because I had an existing
non-"-x" binding that was prefixed with `C-x a`, and when I tried to add
the `C-x a` "-x" binding, it caused the following error:

    bash_execute_unix_command: cannot find keymap for command

Switch the default bindings to `C-x a` and `C-x x` because those are the
ones I originally wanted, since they don't appear to conflict with any
existing default Readline bindings, and they're similar to the Vim
increment and decrement commands.

Now that we have this working, remove the old functions, binding
definitions, and TODOs.

All TODOs here are handled. The old Bash functions and bindings were for
a Bash-only implementation. The point-moving code in
`__readline_incdec_decrement` wasn't applicable to decrementing numbers.

Those caused the following errors:

    Variable "$line" is not imported at -e line 45.
    Variable "$increment_by" is not imported at -e line 45.
    Variable "$point_position" is not imported at -e line 45.
    Variable "$backward" is not imported at -e line 45.
    Global symbol "$line" requires explicit package name (did you forget to declare "my $line"?) at -e line 45.
    Global symbol "$increment_by" requires explicit package name (did you forget to declare "my $increment_by"?) at -e line 45.
    Global symbol "$point_position" requires explicit package name (did you forget to declare "my $point_position"?) at -e line 45.
    Global symbol "$backward" requires explicit package name (did you forget to declare "my $backward"?) at -e line 45.
    Execution of -e aborted due to compilation errors.

I suppose I could have kept warnings, as only strict caused the errors,
but decided to remove it anyway. Not sure what makes the most sense.

Include the built `incdec.pl` code in the Bash file by generating it
from `incdec.pm`.

Previously, point would stay in the same position, causing it to look
like it moved when incrementing or decrementing between a negative
number and a positive number. This moves point by one to the right when
decrementing to a negative integer, or by one to the left when
incrementing to a positive integer. That makes the point seem like it
hasn't moved and making it look more natural.

For example, now point behaves like this:

    $ test 12 0
              ^
    _decrement_

    $ test 12 -1
               ^

    $ test 12 -1
               ^
    _increment_

    $ test 12 0
              ^

Before, it would do:

    $ test 12 0
              ^
    _decrement_

    $ test 12 -1
              ^

    $ test 12 -1
               ^
    _increment_

    $ test 12 0
               ^

If we change between negative and positive numbers, point moves:

    $ test 12 0
               ^
    _decrement_

    $ test 12 -1
               ^

I think point should instead be

    $ test 12 -1
                ^

Not sure that we should be moving point when changing from negative to
positive, though.