Exposes a few new interesting symbols in rust_icu_sys

[filmil]

Some symbols requested were from the 'tools' repo which is not part of the released API, so I added a way to generate those too.

Since this is not relevant for the "regular" rust_icu_sys, I added a separate Makefile target.

Run the following to regenerate:

env ICU_SOURCE_DIR=$HOME/code/icu make static-bindgen-special

The results and tools are in ./rust_icu/rust_icu_sys/bindgen_special

Issue: #264

[filmil]

I'll merge this once (if) the presubmits pass.

[Manishearth]

they've passed!