Compile libformat_parser and link to it.
gcc/rust/ChangeLog:
* Make-lang.in: Compile libformat_parser.
* ast/rust-fmt.cc: New FFI definitions.
* ast/rust-fmt.h: Likewise.
* expand/rust-macro-builtins.cc (MacroBuiltin::format_args_handler): Call
into libformat_parser.
* expand/rust-macro-builtins.h: Define format_args!() handler proper.
libgrust/ChangeLog:
* libformat_parser/Cargo.lock: New file.
* libformat_parser/Cargo.toml: New file.
* libformat_parser/generic_format_parser/Cargo.toml: New file.
* libformat_parser/generic_format_parser/src/lib.rs: New file.
* libformat_parser/src/bin.rs: New file.
* libformat_parser/src/lib.rs: New file.
