Account for front matter when calculating `sourcepos` #1130

Job	Run time
build_bin_spec (nightly)	1m 8s
build_lib_test (nightly)	59s
build_bin_spec (beta)	1m 12s
no_features_build_test (nightly)	32s
build_lib_test (beta)	1m 18s
build_bin_spec (stable)	1m 9s
no_features_build_test (beta)	33s
build_lib_test (stable)	1m 27s
build_bin_spec ($MSRV)	1m 56s
no_features_build_test (stable)	34s
build_lib_test ($MSRV)	2m 17s
no_features_build_test ($MSRV)	1m 6s
build_wasm (nightly)	54s
clippy_format	1m 14s
build_wasm (beta)	54s
lockfile	1m 31s
build_wasm (stable)	45s
build_wasm ($MSRV)	1m 36s
	21m 5s

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