Implement `MallocSizeOf` for cssparser types #341

Job	Run time
Linux (nightly)	2m 32s
Linux (nightly, --features dummy_match_byte)	2m 37s
Linux (nightly, --features malloc_size_of)	2m 34s
Linux (beta)	30s
Linux (beta, --features dummy_match_byte)	31s
Linux (beta, --features malloc_size_of)	32s
Linux (stable)	57s
Linux (stable, --features dummy_match_byte)	24s
Linux (stable, --features malloc_size_of)	26s
Linux (1.63.0)	2m 54s
Linux (1.63.0, --features dummy_match_byte)	2m 44s
Linux (1.63.0, --features malloc_size_of)	2m 47s
Linux (nightly, --features bench)	5m 53s
Linux (nightly, --features bench,dummy_match_byte)	5m 59s
Result	1s
	31m 21s

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