test stable_encodable fails to compile with "missing fragment specifier"

[jayvdb]

cargo test fails to build tests with

error: missing fragment specifier
   --> tests/stable_encodable.rs:120:28
    |
120 |         ($ty_name:ident: $($tail)*)
    |                            ^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
    = note: `#[deny(missing_fragment_specifier)]` on by default

[jayvdb]

Dup of #33