Consistent port and interrupter register API with accessor .set_at(i) syntax

[paulsohn]

Currently, there are two register set arrays(ports and interrupters) but their designs are different and both have several issues:

• Since Allow updating single fields of InterrupterRegisterSet #142, interrupter type became a struct of accessors so that each field can be accessed independently. Port register set is still a struct of data fields and can't be updated field-wise. More consistent design between two register set arrays is required.
• Making new PortRegisterSet to pair with Port (just like InterrupterRegisterSet pairs with Interrupter) is inefficient and violates DRY principle.
• Current indexing method of interrupters (InterrupterRegisterSet::interrupter(&self, i)) actually maps and unmaps 5 physical addresses per indexing. I'm concerned about that if (un)mapping cost is more than zero, this would not be the optimal implementation.

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I've crated a PR(toku-sa-n/accessor#44) for accessor .at(i) and .set_at(i) syntax for this purpose. Applying those changes in xhci crate, I've made a commit but I can't make it a PR until the accessor crate PR has been merged, so I opened this issue.

Changes

• Removed InterrupterRegisterSet (as non-mapping array-like type as it is now) and Interrupter (as struct of accessors)
• The name InterrupterRegisterSet is used for the indexed item type(struct of data field) of IRS array.
• InterrupterRegisterSet and PortRegisterSet both implements accessor::array::BoundSetGeneric trait to use .set_at(i) method.

implementation detail

Internally `accessor::array::BoundSetGeneric` trait is implemented by deriving `BoundSetGenericOfInterrupterRegisterSet` and `BoundSetGenericOfPortRegisterSet` types.

Usecase

• (before)

use xhci::Registers;

// let regs: Registers<M>;
let erdp = regs.interrupter_register_set.interrupter(i).erdp.read_volatile();
let portsc = regs.port_register_set.read_volatile_at(i).portsc;

• (after)

use xhci::Registers;
use xhci::accessor::array::BoundSetGeneric; // for `.set_at(i)` method

// let regs: Registers<M>;
let erdp = regs.interrupter_register_set.set_at(i).erdp.read_volatile();
let portsc = regs.port_register_set.set_at(i).portsc.read_volatile();