Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/src/symbols/rust_slice.rs

@@ -0,0 +1,20 @@
+use crate::rust_slice::RustSlice;
+use core::mem::MaybeUninit;
+use core::ptr::{self, NonNull};
+
+#[export_name = "cxxbridge1$slice$new"]
+unsafe extern "C" fn slice_new(this: &mut MaybeUninit<RustSlice>, ptr: NonNull<()>, len: usize) {
+    let this = this.as_mut_ptr();
+    let rust_slice = RustSlice::from_raw_parts(ptr, len);
+    unsafe { ptr::write(this, rust_slice) }
+}
+
+#[export_name = "cxxbridge1$slice$ptr"]
+unsafe extern "C" fn slice_ptr(this: &RustSlice) -> NonNull<()> {
+    this.as_non_null_ptr()
+}
+
+#[export_name = "cxxbridge1$slice$len"]
+unsafe extern "C" fn slice_len(this: &RustSlice) -> usize {
+    this.len()
+}