add support for emitting casts to and from *mut T and Cell<T>

[aneksteind]

Fixes #964 . The lighttpd-minimal test had failures due to unsupported casts that the rewriter wanted to emit, namely in going to and from *mut T and Cell<T>. This adds support for those casts via:

• an as_ptr() rewrite for Cell<T>, which produces the underlying *mut T
• a *mut T -> *const Cell<T> -> &Cell<T> rewrite, accomplished through a pointer-to-pointer cast followed by a reborrow &*c where c: *const Cell<T>

[kkysen]

The *mut T -> *const T -> *const Cell<T> is safe because Cell is #[repr(transparent)], right?

[aneksteind]

The mut T -> const T -> *const Cell is safe because Cell is #[repr(transparent)], right?

That sounds correct to me