wrapper: Pin blob source insted of unnecessarily RC'ing it

The refcounter here makes little sense as the string is only ever
referenced by DxcIncludeHandlerWrapper.  The count is never artificially
inremented to represent implicit referencing from a native DxcBlob.
Overall lifetime of the string is not altered: as soon as
DxcIncludeHandlerWrapper goes out of scope the underlying string
disappears as well.

Furthermore, use Pin<> to hold the string at the same location in
memory.  In particular this Pin instance only gives us access to the
deref of `String`, which is `&str` and does not allow destructive
operations like resizing or reassignment that could potentially
reallocate heap memory (hence moving the pointer).

src/wrapper.rs

@@ -11,7 +11,7 @@ use com_rs::ComPtr;
 use libloading::{Library, Symbol};
 use std::convert::Into;
 use std::ffi::c_void;
-use std::rc::Rc;
+use std::pin::Pin;
 
 #[macro_export]
 macro_rules! check_hr {
@@ -135,7 +135,7 @@ struct DxcIncludeHandlerWrapperVtbl {
 struct DxcIncludeHandlerWrapper<'a> {
     vtable: Box<DxcIncludeHandlerWrapperVtbl>,
     handler: Box<dyn DxcIncludeHandler>,
-    pinned: Vec<Rc<String>>,
+    pinned: Vec<Pin<String>>,
     library: &'a DxcLibrary,
 }
 
@@ -164,19 +164,18 @@ impl<'a> DxcIncludeHandlerWrapper<'a> {
         let source = unsafe { (*me).handler.load_source(filename) };
 
         if let Some(source) = source {
-            let pinned_source = Rc::new(source);
-
+            let source = Pin::new(source);
             let mut blob = unsafe {
                 (*me)
                     .library
-                    .create_blob_with_encoding_from_str(pinned_source.as_str())
+                    .create_blob_with_encoding_from_str(&source)
                     .unwrap()
             };
 
             unsafe {
                 blob.inner.add_ref();
                 *include_source = *blob.inner.as_mut_ptr();
-                (*me).pinned.push(pinned_source);
+                (*me).pinned.push(source);
             }
 
             0