yaml-rust2 0.13.0: flow mapping inside a flow-sequence pair is split at its first comma

With yaml-rust2 0.13.0 (latest on crates.io), [z: {b, c}] loads as [{z: {b: null}, c: null}] instead of [{z: {b: null, c: null}}], and [z: &a [b, c]] and [y: {'': }] are rejected. The cause is that the scanner's flow_mapping_started / implicit_flow_mapping are single bools, not per flow level, so the comma inside the nested collection ends the outer implicit pair. saphyr fixed the same thing on master (keep both states in a Vec pushed at [/{ and popped at the closer), but the fix was never ported to yaml-rust2. The same port passes yaml-test-suite 402/402. Same release: a: |\n\n at end of input gives "\n" instead of "" (saphyr fixed that too).