c_str: add `.as_ptr` & `.as_mut_ptr` to replace `.with_[mut_]ref`.

These forms return the pointer directly, rather than the added
indirection, indentation, and inefficiencies of the closures in
`.with_ref` and `.with_mut_ref`. The two closure functions are
deprecated.

Replace

    foo(c_str.with_ref(|p| p))

    c_str.with_ref(|p| {
        foo(p);
        bar(p);
    })

with

    foo(c_str.as_ptr())

    let p = c_str.as_ptr();
    foo(p);
    bar(p);

This change does mean that one has to be careful to avoid writing `let p
= x.to_c_str().as_ptr();` since the `CString` will be freed at the end
of the statement. Previously, `with_ref` was used (and `as_ptr` avoided)
for this reason, but Rust has strongly moved away from closures to more
RAII-style code, and most uses of `.with_ref` where in the form
`.with_ref(|p| p)` anyway, that is, they were exactly `.as_ptr`.

[breaking-change]
c_str: replace .with_ref with .as_ptr throughout the codebase.
c_str: move `unwrap` & document it more clearly.

This should be called rarely, but it was placed first in the list of
methods, making it very tempting to call.
auto merge of #14904 : huonw/rust/cstr-remove-withref, r=alexcrichton