Fixes Rust code generation for single file output when using namespaces (#8877)

* Adds tests for Rust single file mode

All existing tests only compile Rust code using --rust-module-root-file.

* Adds standalone include tests for Rust

The imports for these tests have been moved to their own file, as the existing intergration_test.rs file hides compilation issues from code generation due to symbols brought into scope outside of the generated code (e.g. `extern crate alloc`).

* Declare alloc crate in every Rust namespace

When performing code generation within a single file, extern crate alloc needs to be delcared to bring alloc into scope within every inner namespace.

* Regenerates generated schemas

src/idl_gen_rust.cpp

@@ -424,7 +424,6 @@ class RustGenerator : public BaseGenerator {
     code_.Clear();
     code_ += "// " + std::string(FlatBuffersGeneratedWarning());
     code_ += "// @generated";
-    code_ += "extern crate alloc;";
 
     assert(!cur_name_space_);
 
@@ -3006,6 +3005,7 @@ class RustGenerator : public BaseGenerator {
           "use self::serde::ser::{Serialize, Serializer, SerializeStruct};";
       code_ += "";
     }
+    code_ += "extern crate alloc;";
   }
 
   // Set up the correct namespace. This opens a namespace if the current