Add support for subscript extension #1082

Job	Run time
clippy_format	2m 5s
build_lib_test (nightly)	1m 2s
build_lib_test (beta)	1m 4s
build_lib_test (stable)	1m 4s
build_wasm (nightly)	47s
no_features_build_test (nightly)	35s
build_lib_test ($MSRV)	2m 52s
lockfile	1m 3s
build_wasm (beta)	50s
no_features_build_test (beta)	30s
build_wasm (stable)	46s
no_features_build_test (stable)	25s
build_wasm ($MSRV)	2m 16s
build_bin_spec (nightly)	1m 10s
no_features_build_test ($MSRV)	1m 53s
build_bin_spec (beta)	1m 10s
build_bin_spec (stable)	4m 14s
build_bin_spec ($MSRV)	2m 51s
	26m 37s

Provide feedback