oiiotool: control flow

How do you craft an oiiotool command that will examine a file and add
an alpha=1 channel if it's a 3-channel file, but leave the existing
alpha alone if it's already a 4 channel file?

This patch adds control flow, so you could express the above like this:

    oiiotool in.exr --if "{TOP.nchannels < 4}" --ch R,G,B,A=1 --endif -o out.exr

The following slate of new fetures is added:

* `--set name value` sets a "user variable".

* Expression evaluation will expand identifiers that are the names of
  user variables. For example, `{i}` is replaced by the value of user
  variable `i`, and `{i+3}` returns 3 more than the current numerical
  value of `i`.

* Expression evaluation has been extended to support the following
  numerical comparison operators: `<` `>` `<=` `>=` `==` `!=` and
  `<=>` (the last one being the 3-way comparison, like in C++20).
  Note that the comprison operators have the same precedence as `+`
  and `-`, so use parentheses liberally to disambiguate complex mixed
  expressions.

* `--if condition commands... [--else commands...] --endif`

* `--while condition commands... --endwhile`

* `--for variable range commands... --endfor` where the range may be,
  like Python, any of `end` (implied begin=0, step=1), `begin,end`
  (implied step=1), or `begin,end,step`.

Also needed to modify ArgParse so that it has the notion of whether
it's "running" -- actually calling the actions for each argument, or
if it's merely parsing but taking no action -- and to be able to set
and retrieve which command line argument index an action is responding
to.

Author: lgritz

src/cmake/testing.cmake

@@ -155,7 +155,7 @@ endmacro ()
 macro (oiio_add_all_tests)
     #   Tests that require oiio-images:
     oiio_add_tests (gpsread
-                    oiiotool oiiotool-attribs  oiiotool-copy
+                    oiiotool oiiotool-attribs oiiotool-copy
                     oiiotool-xform
                     maketx oiiotool-maketx
                     misnamed-file
@@ -172,7 +172,7 @@ macro (oiio_add_all_tests)
     oiio_add_tests (
                     cmake-consumer
                     nonwhole-tiles
-                    oiiotool-composite
+                    oiiotool-composite oiiotool-control
                     oiiotool-fixnan
                     oiiotool-pattern
                     oiiotool-readerror