rust: macros: take string literals in module!

Instead of taking binary string literals, take string ones instead,
making it easier for users to define a module, i.e. instead of
calling `module!` like:

    module! {
        ...
        name: b"rust_minimal",
        ...
    }

now it is called as:

    module! {
        ...
        name: "rust_minimal",
        ...
    }

Module names, aliases and license strings are restricted to
ASCII only. However, the author and the description allows UTF-8.

For simplicity (avoid parsing), escape sequences and raw string
literals are not yet handled.

Link: https://github.com/Rust-for-Linux/linux/issues/252
Link: https://lore.kernel.org/lkml/YukvvPOOu8uZl7+n@yadro.com/
Signed-off-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

samples/rust/rust_print.rs

@@ -7,10 +7,10 @@ use kernel::prelude::*;
 
 module! {
     type: RustPrint,
-    name: b"rust_print",
-    author: b"Rust for Linux Contributors",
-    description: b"Rust printing macros sample",
-    license: b"GPL",
+    name: "rust_print",
+    author: "Rust for Linux Contributors",
+    description: "Rust printing macros sample",
+    license: "GPL",
 }
 
 struct RustPrint;