[FIX][RUST] use $crate:: in tvm_ffi_dll_export_typed_func! and ensure!

[lucifer1004]

Summary

Three small bugs in the Rust ergonomics that prevented the macros from
being usable from downstream cdylibs:

1. ensure! macro expanded to crate::bail!, which resolves to
   the caller crate at expansion site rather than tvm_ffi. Switched
   to $crate::bail! so the path resolves correctly in any crate.

2. tvm_ffi_dll_export_typed_func! referenced
   tvm_ffi_sys::TVMFFIAny without a $crate:: prefix, forcing every
   downstream crate to add tvm-ffi-sys to its own [dependencies].
   Switched to $crate::tvm_ffi_sys::TVMFFIAny; downstream crates now
   only need to depend on tvm-ffi.

3. Missing #[no_mangle]: the generated
   pub unsafe extern "C" fn __tvm_ffi_<name> had no #[no_mangle],
   so the linker stripped the symbol from cdylibs and
   Module::GetFunction could not find it. Added
   #[unsafe(no_mangle)] (supported in 2021 + 2024 editions on
   rustc ≥ 1.82).

Reproduction (before)

Building a downstream cdylib that uses tvm_ffi_dll_export_typed_func!:

```text
error[E0433]: cannot find module or crate `tvm_ffi_sys` in this scope
= note: this error originates in the macro `tvm_ffi_dll_export_typed_func`
```

After adding `tvm-ffi-sys` as a direct dep:

```text
error[E0433]: cannot find `bail` in the crate root
= note: this error originates in the macro `tvm_ffi::ensure`
```

After working around `ensure!`, the cdylib builds — but
`nm -D libfoo.so | grep tvm_ffi` returns nothing because
`#[no_mangle]` is missing.

Test plan

• Built a downstream cdylib that only depends on `tvm-ffi` (no
  `tvm-ffi-sys` direct dep).
• Loaded via `tvm_ffi.load_module(...)` from Python.
• Called three typed functions: scalar (`sum_i32`), 1-D tensor
  (`add_one_inplace`), 2-tensor (`dot_f32`). All round-trip.
• `nm -D libfoo.so | grep __tvm_ffi` shows the exported symbols.

Part 1 of 2 — companion PR is #591 (Module::LoadFromBytes).
The second adds `Module::LoadFromBytes`.

[gemini-code-assist]

Code Review

This pull request improves macro hygiene in rust/tvm-ffi by using $crate for internal references and adds the no_mangle attribute to exported FFI functions to ensure they are preserved in the output binary. A review comment points out that #[unsafe(no_mangle)] is a recent Rust feature (1.82.0+) and suggests using the standard #[no_mangle] to avoid an unnecessary increase in the Minimum Supported Rust Version (MSRV).

[gemini-code-assist]

The #[unsafe(no_mangle)] attribute is a relatively new feature introduced in Rust 1.82.0. Using it will cause compilation errors for any users or downstream crates using an older version of the Rust compiler (e.g., 1.81 or earlier). Unless this project has a strict requirement to bump its Minimum Supported Rust Version (MSRV) to 1.82, it is safer to use the standard #[no_mangle] attribute, which is compatible with all Rust versions and remains perfectly valid.

Suggested change

	#[unsafe(no_mangle)]
	#[no_mangle]

[lucifer1004]

Thanks @gemini-code-assist — good catch. Switched to plain #[no_mangle] in 7232d83 so the macro stays compatible with rustc < 1.82. Edition-2024 callers will see a deprecation warning, which is harmless; if/when this project bumps MSRV to 1.82+, the attribute can be migrated to #[unsafe(no_mangle)] repo-wide in one go.

[gemini-code-assist]

Thanks for the update, @lucifer1004. That approach makes sense for maintaining compatibility with older toolchains while preparing for future MSRV updates. The changes look correct and address the issues described in the PR summary.