[phobos] std.math.abs + user-defined types

[David Simcha]

  std.math.abs looks to have been written to accommodate user-defined 
numeric types, but then have had pure nothow stuck on it somewhat 
haphazardly.  IMHO now that Phobos is starting to grow some user-defined 
numeric types, abs() should work with them.  Other std library functions 
should, too, as much as practical.  IMHO the best solution for this 
specific case, since the code is so trivial, is to just make two 
versions:  One that's  pure nothrow and accepts builtin numeric types, 
and one that's not pure nothrow and accepts user-defined types.  
(Multiplying a BigInt by -1, for example, probably can't easily be made 
pure.)  Should this quick fix be done for now so that abs() works with 
BigInt and Rational if/when it's checked in?

On the other hand, the problem of propagating purity for generic 
functions probably needs to be addressed on a more general level.  I 
think this is easily solvable and I'm going to propose a solution on the 
newsgroup momentarily.

--Dave