extendr-api: add as_str_mut

this might not make sense for NA strings
for now, there is a `todo!`

extendr-api/src/robj/mod.rs

@@ -593,6 +593,7 @@ impl Robj {
         unsafe {
             let charsxp = match self.sexptype() {
                 STRSXP => {
+                    // only allows scalar strings
                     if self.len() != 1 {
                         return None;
                     }
@@ -613,6 +614,42 @@ impl Robj {
             Some(std::str::from_utf8_unchecked(all_bytes))
         }
     }
+    /// Get a mutable reference to a scalar string type.
+    /// ```
+    /// use extendr_api::prelude::*;
+    /// test! {
+    ///    let robj1 = Robj::from("xyz");
+    ///    let robj2 = Robj::from(1);
+    ///    assert_eq!(robj1.as_str(), Some("xyz"));
+    ///    assert_eq!(robj2.as_str(), None);
+    /// }
+    /// ```
+    pub fn as_str_mut<'a>(&mut self) -> Option<&'a mut str> {
+        unsafe {
+            let charsxp = match self.sexptype() {
+                STRSXP => {
+                    // only allows Scalar strings
+                    if self.len() != 1 {
+                        return None;
+                    }
+                    Some(STRING_ELT(self.get_mut(), 0))
+                }
+                CHARSXP => Some(self.get_mut()),
+                SYMSXP => Some(PRINTNAME(self.get_mut())),
+                _ => None,
+            };
+
+            let charsxp = charsxp?;
+            if charsxp == R_NaString {
+                todo!()
+                // return Some(<&str>::na());
+            }
+
+            let length = Rf_xlength(charsxp);
+            let all_bytes = std::slice::from_raw_parts_mut(R_CHAR(charsxp) as _, length as _);
+            Some(std::str::from_utf8_unchecked_mut(all_bytes))
+        }
+    }
 
     /// Get a scalar integer.
     /// ```