Porting simple_pv to rust.
- Added a simple_pv_rs_sys to link with the C interface
- Added simple_pv_rs for a higher level interface
- Supporting uint16, int32, float32, float64, and string types
- Extending the simple_epics code to use atomic reads/writes for integer values.
- Extending the simple_epics code to specify read/write functions to deal with string values not being atomic.