Add registers::debug (#286)

* Add registers::debug

* impl DebugAddressRegister in macro

* Declare debug register acesses as nomem

* Use u64 for DebugAddressRegisters

* Dr7: Remove get_ prefix from getters

* debug.rs: Remove redundant denying of unsafe_op_in_unsafe_fn

* debug.rs: Remove TryFrom impls

They don't add value over the inherent constructor functions.

* Don't set repr on DebugAddressRegisterNumber

* Make DebugAddressRegisterNumber an enum

* debug.rs: Remove redundant asm import

* Add DebugAddressRegister::NUM

* Strip Hw Prefix from HwBreakpointCondition and HwBreakpointSize

Author: mkroening