Scene storage backend integration (#51)

This PR adds Storyteller.ai API integration for saving and loading
Storyteller Studio scenes to/from the backend.

### Known issues
* The skybox is not currently stored in the serialized scene format

studio/src/path.rs

@@ -0,0 +1,40 @@
+use std::{ffi::OsStr, path::Path};
+
+pub trait PathExtras {
+    /// See [std::path::Path::file_prefix]
+    #[must_use]
+    fn ext_file_prefix(&self) -> Option<&OsStr>;
+}
+
+impl PathExtras for Path {
+    fn ext_file_prefix(&self) -> Option<&OsStr> {
+        self.file_name()
+            .map(split_file_at_dot)
+            .map(|(before, _after)| before)
+    }
+}
+
+/// Note: Copied from private fn `std::path::split_file_at_dot`
+fn split_file_at_dot(file: &OsStr) -> (&OsStr, Option<&OsStr>) {
+    let slice = file.as_encoded_bytes();
+    if slice == b".." {
+        return (file, None);
+    }
+
+    // The unsafety here stems from converting between &OsStr and &[u8]
+    // and back. This is safe to do because (1) we only look at ASCII
+    // contents of the encoding and (2) new &OsStr values are produced
+    // only from ASCII-bounded slices of existing &OsStr values.
+    let i = match slice[1..].iter().position(|b| *b == b'.') {
+        Some(i) => i + 1,
+        None => return (file, None),
+    };
+    let before = &slice[..i];
+    let after = &slice[i + 1..];
+    unsafe {
+        (
+            OsStr::from_encoded_bytes_unchecked(before),
+            Some(OsStr::from_encoded_bytes_unchecked(after)),
+        )
+    }
+}