field-offset: safe pointer-to-member functionality

This crate implements an offset_of!(...) macro which safely encapsulates a pointer-to-member.

Example: ```rust struct Foo { x: u32, y: f64 }

let fooy = offsetof!(Foo => y);

let mut a = Foo { x: 1, y: 2.0 };

*fooy.applymut(&mut a) = 3.0;

assert!(a.y == 3.0); ```

The macro returns an instance of FieldOffset<T, U>, which represents a pointer to a field of type U within a containing type, T.

The FieldOffset type implements Add. Applying the resulting field offset is equivalent to applying the first field offset, then applying the second field offset.

The macro also supports accessing nested fields:

rust let bar_foo_y = offset_of!(Bar => foo: Foo => y);