Fully generic sysvar implementations

[tgross35]

I think I actually came up with a way to make sysvar implementations fully generic using struct update syntax. Proof:

struct St {
    x: *mut i32,
    y: i32
}

unsafe impl Sync for St {}

trait Foo {
    const DEFAULT_ST: St = St{x: std::ptr::null_mut(), y: 0};
}

struct Bar;

impl Foo for Bar {}

static TEST: St = St {x: 0x123 as *mut i32, ..Bar::DEFAULT_ST};

The proc macro would create something like TEST. Just needs to detect string literals and convert them to c strings, and keep a map of field mappings.

To keep safety, 1. this trait needs to be marked unsafe and 2. our proc macro should check and reject raw pointer