Floating point rounding modes: we should restrict them slightly

[Don]

#ponce wrote:
> Why not imposing a rounding mode (ex :  toward minus infinity) ?
> And then disallowing to change it in assembly.

You can't impose a rounding mode in a system language: rounding mode is 
a crucial feature in a few rare but very important cases. The trick is 
to allow those cases without polluting everything else. This tiny 
proposal gets rid of most the damage.

However, my previous proposal for fixing the interaction of 'pure' with 
rounding modes and exception handling, would allow the rounding mode to 
be round-to-nearest almost everywhere, and changeable only in impure 
functions inside modules marked module(advancedfloatingpoint). This 
could give performance benefits everywhere outside those modules.