Rust: Work around 'error[E0658]: `let...else` statements are unstable'

Rust: Work around 'error[E0658]: `let...else` statements are unstable'

Compiling with Debian GNU/Linux 12 (bookworm) packages:

    $ apt-cache madison cargo rustc
         cargo | 0.66.0+ds1-1 | http://deb.debian.org/debian bookworm/main ppc64el Packages
         cargo | 0.66.0+ds1-1 | http://deb.debian.org/debian bookworm/main Sources
         rustc | 1.63.0+dfsg1-2 | http://deb.debian.org/debian bookworm/main ppc64el Packages
         rustc | 1.63.0+dfsg1-2 | http://deb.debian.org/debian bookworm/main Sources

..., we run into:

       Compiling generic_format_parser v0.1.0 ([...]/source-gcc/libgrust/libformat_parser/generic_format_parser)
    error[E0658]: `let...else` statements are unstable
       --> generic_format_parser/src/lib.rs:994:5
        |
    994 | /     let Some(unescaped) = unescape_string(snippet) else {
    995 | |         return InputStringKind::NotALiteral;
    996 | |     };
        | |______^
        |
        = note: see issue #87335 <https://github.com/rust-lang/rust/issues/87335> for more information

Rewrite backwards, per <https://rust-lang.github.io/rfcs/3137-let-else.html>.

	libgrust/
	* libformat_parser/generic_format_parser/src/lib.rs: Work around
	'error[E0658]: `let...else` statements are unstable'.