Update Interoperability section

Following the discussion on #348, this PR proposes the following changes to the Interoperability section:

Update descriptions regarding the availability of core::ffi.
Remove mentions of std::os::raw.

I intentionally kept mentions of the cty crate and the cstr_core crate since the next section uses the former. We may also consider updating the following section to use core::ffi instead of the cty crate.

This PR also introduces two additional changes.

First, it changes the descriptions regarding the implicit conversion between C and Rust types. The book says that the compiler implicitly converts between the two types, but it doesn't sound right to say that the example (reproduced below) works because of the conversion. It works simply because, on platforms where unsigned int is 32-bit long, c_uint and u32 are the same type because the former is a type alias of the latter. This PR also removes the unsafe from the example since I believe it plays no role.

unsafe fn foo(num: u32) {
    let c_num: c_uint = num;
    let r_num: u32 = c_num;
}

Second, it changes the text formatting in the type correspondence table. The types in the table now use code formatting. Also, the presentation of C pointer types is updated (e.g. *char → char *).

Closes #348.

rust-embedded / book

Update Interoperability section #351