Make all proc macros optional #426

Job	Run time
Linux (stable, --no-default-features)	29s
Linux (beta)	36s
Linux (stable)	34s
Linux (nightly)	2m 6s
Linux (beta, --no-default-features)	34s
Linux (1.71.0, --no-default-features)	32s
Linux (nightly, --features bench)	5m 25s
Linux (stable, --features malloc_size_of)	36s
Linux (nightly, --no-default-features)	1m 58s
Linux (1.71.0, --features malloc_size_of)	37s
Linux (beta, --features malloc_size_of)	37s
Linux (1.71.0)	39s
Linux (nightly, --features malloc_size_of)	2m 10s
Result	2s
	16m 55s

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