fix(insert sig lifetime): key and value should have different lifetimes

[zjp-CN]

also add generic_signature_lifetimes test for it

---

For the latest version, the write_key_generic function won't compile:

error[E0597]: `buf` does not live long enough
  --> src/main.rs:27:31
   |
20 |         key: K::SelfType<'_>,
   |         --- has type `<K as RedbValue>::SelfType<'1>`
...
23 |         let buf = [1, 2, 3];
   |             --- binding `buf` declared here
...
27 |             table.insert(key, buf.as_slice()).unwrap();
   |             ------------------^^^^^^^^^^^^^^-
   |             |                 |
   |             |                 borrowed value does not live long enough
   |             argument requires that `buf` is borrowed for `'1`
...
30 |     }
   |     - `buf` dropped here while still borrowed

[cberner]

Looks good to me, except one minor thing