librust: Stop rust tool from crashing on macos.

#7617 

While the code that was there should've been perfectly fine (and seemingly is on linux at least) there seems to be some sort of weird interaction going on with statics and vectors. I couldn't get a smaller test case to reproduce that behaviour. The for loop in `rust::usage` seemingly just goes past the end of the vector thus getting garbage which it tries to pass to malloc somewhere down the line.

In any case, using a fixed length vector seems to mitigate this.