Rust: libformat_parser: Lower minimum Rust version to 1.49

libgrust/ChangeLog:

	* libformat_parser/Cargo.toml: Change Rust edition from 2021 to 2018.
	* libformat_parser/generic_format_parser/Cargo.toml: Likewise.
	* libformat_parser/generic_format_parser/src/lib.rs: Remove usage of
	then-unstable std features and language constructs.
	* libformat_parser/src/lib.rs: Likewise, plus provide extension trait
	for String::leak.