remove panics from library code

[rillian]

There are a number of unwrap or expect assertions in the STAR implementation. Where these handle input coming from outside the library--n particular user or untrusted network data--it's important to report an error instead so the calling application can handle it gracefully rather than crashing.

Triggering a panic is fine on locally-controlled input, or in test code. Other use should be audited and converted to a Result return value.