Add checked arithmetic functions to addresses, pages, and frames

[mkroening]

This adds checked_add and checked_sub const functions corresponding to the current Add and Sub implementations for:

• VirtAddr
• PhysAddr
• Page
• PhysFrame
• MappedPage (not public)
• MappedPageItem (not public)

Note that this changes the panic behavior for Page and PhysFrame, since rhs * S::SIZE did not panic before without overflow checks, such as in the release profile.

Also note that this PR does not add a checked version for subtracting two addresses, two pages, or two frames yet, but these would require a different function name. I don't require them personally at the moment. Would you like me to add them anyway? They could be called checked_offset_from, similar to the corresponding pointer types functions in std. If you prefer, we can also defer this. Adding such methods should not be breaking.

This PR is needed for #617 for #611. The approach was discussed in #611 (comment).

This PR is best reviewed commit by commit.

Fixes #293.

[mkroening]

CI just made me notice #[const_fn(cfg(not(feature = "memory_encryption")))] on PhysAddr::new().

This is really unfortunate, since any crate in the crate graph that enables this feature makes other code in other crates not compile anymore. I don't know if we have any perspective on fixing that and whether we should open an issue for that.

[mkroening]

I just noticed #293. That issue proposes traits. This PR adds inherent methods instead, as is done for integers in the standard library. What would you prefer? I'll mark this PR as closing that issue. Please let me know if I should undo that.