noexcept all the things

[lgritz]

This patch marks as noexcept absolutely everything that could be under
C++11 rules. Excluded are, obviously, anything that intentionally throws,
and those are very few after Owen's recent Imath overhaul and my changes
to eliminate the weird integer versions of normalize et al.

This patch also makes most of numeric_limits constexpr, per
C++11 rules. But a few are not constexpr yet because parts of half
still need to be fully constexpr-plumbed, and there are some tricky
parts (and maybe not 100% possible in C++11). I may revisit this in a
later patch.

Using noexcept is a promise that the caller to your function won't need
to handle exceptions. If your function calls something that turns out to
throw, it's on you to catch it, or else the application will crash with
an uncaught exception.

It's important that this get a thorough review. I really need a few
keen eyes to make sure that I haven't inadvertently marked anything
noexcept if it calls things (including outside Imath itself) that
might throw exceptions. (I mean, we can fix those bugs later, but I'd
prefer to find them now rather than in the heat of production.)

Closes #64

Signed-off-by: Larry Gritz lg@larrygritz.com

[meshula]

This is one of those diffs that seems to need one of the specialized Japanese "thank you" variants, like gokorusama deshita, for use on completion of a mundane but necessary task with a significant amount of work expended but no particular glamour... Which is to say, thanks for taking on the task!

I tried to find potential issues, such as nan's in calculations on frustums with a zero fov, and I didn't spot any problems.

[lgritz]

I'll give this through the weekend for anybody else to inspect and/or complain, will merge next week if I haven't seen any objections.