Add support for subscript extension #1100

Job	Run time
clippy_format	2m 33s
build_bin_spec (nightly)	1m 7s
build_wasm (nightly)	52s
build_bin_spec (beta)	1m 32s
build_lib_test (nightly)	59s
build_wasm (beta)	48s
lockfile	56s
build_bin_spec (stable)	1m 1s
build_wasm (stable)	50s
build_bin_spec ($MSRV)	3m 3s
build_wasm ($MSRV)	2m 37s
no_features_build_test (nightly)	30s
build_lib_test (beta)	1m 25s
no_features_build_test (beta)	31s
build_lib_test (stable)	1m 17s
no_features_build_test (stable)	32s
build_lib_test ($MSRV)	3m 39s
no_features_build_test ($MSRV)	2m 23s
	26m 35s

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