deny(missing_docs) at crate root

To help flag missing documentation on publicly accessible APIs.
diff --git a/src/cxx_string.rs b/src/cxx_string.rs
index dce7053..126acb1 100644
--- a/src/cxx_string.rs
+++ b/src/cxx_string.rs
@@ -213,6 +213,7 @@
     space: MaybeUninit<[usize; 8]>,
 }
 
+#[allow(missing_docs)]
 impl StackString {
     pub fn new() -> Self {
         StackString {