Additional macros for rocket.rs

Derive FromRequest

```rust use rocketextracodegen::FromRequest;

[derive(FromRequest)]

struct BookingService { dbconn: DbConnection, queueconn: QueueConnection, user: AuthUser, } ```

The errors of the individual fields will be converted via the From trait to a common target error type. The target error type defaults to () but can be overridden manually:

```rust use rocketextracodegen::FromRequest;

[derive(FromRequest)]

[error_type = "MyError"]

struct BookingService { dbconn: DbConnection, queueconn: QueueConnection, user: AuthUser, } ```