implement @byteOffsetOf and @bitOffsetOf in userland

[andrewrk]

At runtime you can do this to get offset:

    var a: A = undefined;
    assert(@ptrToInt(&a.a) - @ptrToInt(&a) == @byteOffsetOf(A, "a"));

We could make this work at compile time with the concept of lazy evaluated comptime values. For example even though we don't know the actual integer address of &a at comptime or of &a.a, if the only thing the comptime code does with these values is subtract them, we can have that return the offset. That means we can delete @byteOffsetOf and make it a userland function.

As for @bitOffsetOf something similar could be done with taking the address of a bit packed field, plus @typeInfo, since the bit offset information is in the pointer type.

[andrewrk]

This is superseded by #8642.