unraw the identifiers

Fixes https://github.com/eqrion/cbindgen/issues/410

tests/rust/raw_ident.rs

@@ -0,0 +1,20 @@
+#[repr(u8)]
+pub enum r#Enum {
+    r#a,
+    r#b,
+}
+
+#[repr(C)]
+pub struct r#Struct {
+    r#field: r#Enum,
+}
+
+#[no_mangle]
+pub extern "C" fn r#fn(r#arg: r#Struct) {
+    println!("Hello world");
+}
+
+pub mod r#mod {
+    #[no_mangle]
+    pub static r#STATIC: r#Enum = r#Enum::r#b;
+}