Currently iostream support is unconditionally enabled. This is fine for desktop targets but not for small embedded targets with limited flash storage and RAM. For instance on the Nordic nRF51 based on ARM Cortex-M0+, approx 150kB flash and 6.5kB of extra RAM is used, on a device which only has 256kB flash and 8kB RAM.

Disabling iostream support will go a long way in supporting embedded targets.

The following patch (against v2.2.x branch) adds a #define to enable/disable iostream:
https://gist.github.com/pvaibhav/c3703bf23c8513f4a8160eed524a415d

I've verified disabling iostream results in zero impact on the compiled binary size or RAM usage.

I'll create a pull request from this patch if there is any interest. My only concern is the duplication of UNIT_ADD definition, once with and once without iostream. An intermediate macro could perhaps help here.