128 bit signed and unsigned integer types

[Walter Bright]

Andrei Alexandrescu wrote:
> If 128-bit built-in integer can't be made more efficient by moving them 
> in the core, then the question is really "do you need 128-bit literals?" 
> because that's all the built-in feature would bring over a library.

The literals would be an issue, otherwise you get into the std::string 
problem where:

s + "abc"

works, but:

"abc" + "def"

does not.

If it's not built-in, you also lose the constant folding and math 
identity stuff.