FixedGattValue (and other ble traits) should be provided by a separate library

[Nashenas88]

I'm unable to write unit tests to validate that my impls for FixedGattValue are correct, since the library can only be compiled for nrf hardware. Splitting the ble traits into their own library would make this possible. I currently have to create a new trait that I can use for testing, then use a newtype wrapper to proxy the implementation for FixedGattValue.