[EXP] use proptest for generating values for tests

[luizirber]

Still not sure if this is a good idea, but wanted to try it out =]

proptest is a crate for property-based testing. In this example, I'm using it to generate values for the specvol function, and then comparing the output of the C and Rust versions by going thru the C API (a test oracle)

This might not be a good idea because:

• each test case generated needs to be compiled (large overhead)
• I'm passing the values as strings and parsing in C using strtof