Use a line_offset vector to track offsets for inline sourcepos #994

Job	Run time
build_bin_spec (nightly)	1m 6s
build_lib_test (nightly)	51s
build_bin_spec (beta)	1m 39s
no_features_build_test (nightly)	31s
build_lib_test (beta)	1m 37s
build_bin_spec (stable)	1m 9s
build_wasm (nightly)	51s
build_lib_test (stable)	1m 15s
no_features_build_test (beta)	35s
build_bin_spec ($MSRV)	3m 51s
build_wasm (beta)	47s
build_lib_test ($MSRV)	3m 48s
no_features_build_test (stable)	29s
build_wasm (stable)	1m 11s
no_features_build_test ($MSRV)	2m 51s
build_wasm ($MSRV)	3m 13s
clippy_format	3m 0s
lockfile	1m 43s
	30m 27s

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