Make i128 support optional

[dbeckwith]

In a similar vein to rust-num/num-traits#177 (fixed by rust-num/num-traits#185), it would be great if support for i128 could be made optional using a cargo feature. The new i128 feature would enable num-traits/i128 and num-integer/i128, and any code in num-rational using the i128 type would need to be conditionally compiled. Basically, add the same kind of i128 feature detection that num-traits and num-integer use, where it turns on the feature if either the cargo feature is enabled, or the probe detects i128 support.

[cuviper]

The current num-rational uses features = ["i128"] unconditionally for those dependencies, which should bypass probing if you're using at least num-traits 0.2.13 and num-integer 0.1.44. Do you find otherwise?

[dbeckwith]

Ah, never mind! I was specifying versions for num-rational and num-traits, but had not updated num-integer in my Cargo.lock. On the latest versions of all of those it seems to work. Sorry for the noise.