Use dedicated types to statically check for write or exec MappedPages permissions

[kevinaboos]

Expand the existing MappedPages type to have statically-typed permissions semantics:

• MappedPages is read-only.
  • It only offers the as_type() and as_slice() functions.
• MappedPagesMut is an extension that is returned when the mapping is created with WRITABLE page table entry flags is set.
  • It adds the as_type_mut() and as_slice_mut() functions.
• MappedPagesExec is another extension that is returned when the mapping is created with the EXEC flag set, or rather when the NO_EXEC flag is not set (which is how it works on x86_64).
  • It adds the as_func() function.

Benefits

This will forbid creating a mapping that is both executable and writable, which is a good thing for security reasons.

This will also help disambiguate between different types of MappedPages* objects that are passed around the system; for example, it will distinguish between the following:

• executable MappedPages (now MappedPagesExec) objects used for .text sections
• read-only MappedPages objects used for .rodata, .gcc_except_table, and .eh_frame sections
• writable MappedPages (now MappedPagesMut) objects used for .data and .bss sections

Finally, this has the obvious benefit of transforming runtime page table entry permissions mismatches into compile-time type mismatches. Currently, the existing singular MappedPages type

Note: this may also allow us to reduce the space occupied by a MappedPages* object by removing the flags field from the struct, but I'm not 100% confident that we can do that yet. I'd need to fully explore all use cases of the flags field in order to determine whether static multi-typing of MappedPages* objects will render the flags field obsolete.

[kevinaboos]

We can actually take this further: by leveraging const generics, we can create MappedPages* instances parameterized with a specific size (in bytes or pages), which is known at compile time. Then we can omit bounds and type size checks for those types when overlaying a struct atop it using as_type().