Support fast math flags

[elliottslaughter]

See #529 (comment) for why this matters. This is the last major blocker (as far as I can tell) on #471.

LLVM fast math flags are documented here. The main question is how to expose them.

Two options that come to mind are adding a function-level attribute exposed by a setter. E.g.:

my_func:setfastmath(true) -- set all flags
my_func:setfastmath("contract") -- set one flag
my_func:setfastmath({"contract", "nnan"}) -- set two flags

This is potentially obnoxious because you need to remember to set it on any functions you care about.

A more whole-program approach would be to set this through terralib.saveobj, probably hijacking the optimize parameter:

terralib.saveobj(filename, filetype, functable, arguments, target, {fastmath = true})

A final option would be to add a flag like:

--fast-math <options>

Which would then be truly whole-program.

[elliottslaughter]

#531 has landed, which adds the saveobj form of this option. I think that's the best approach, but we could potentially add others as well in the future, as they don't need to be mutually exclusive. I've confirmed we get the expended performance from the new version in the apps I've tested.