Exceptions enum and table

[romancardenas]

I added an exception type enum and an exception handler table following the interrupts' fashion.

• The standard exception types are defined in SiFive's interrupt cookbook.
• For the exception handlers, I used Option<unsafe extern "C" fn(&TrapFrame)> instead of a union struct. The Rust compiler provides a niche optimization to represent None as 0 instead of adding an additional byte. This way is more rustacean than using a union and it is FFI safe.
• By default, all the exception handlers are set to ExceptionHandler, so this PR should not break previous works that depended on a single handler.