Revisit wrapper API and check for correct lifetimes #125

We should investigate making the wrapper API more ergonomic and error-resilient. There is some cruft having to do with raw pointers and some internal APIs that become error prone, such as in nj-core/src/basic.rs

pub fn unwrap<T>(&self, js_this: napi_value) -> Result<&'static T, NjError> { }

None of the values involved at the start of this invocation actually live for the full lifetime of the program, so there is the potential for memory unsoundness if it is not managed correctly. We need to craft the types with proper lifetimes that match the semantics of the node napi object lifetimes so that the Rust compiler will correct us on memory management.