prefix all functions with underscore

not sure you'll want to accept this pull request, as it breaks any old code. However, I had some conflicts with the function names with `/usr/include/complex.h` header which is a part of the simulator 7.1 sdk:

Xcode was pointing to this section as a conflict and refusing to compile:
```
/*  Constant expression of type const float _Complex                          */
#define _Complex_I (__extension__ 1.0iF)
#undef I
#define I _Complex_I
```