feat(core): add access methods for `Buffer` #675

Job	Run time
check_docs	2m 53s
check_clippy	3m 17s
build_all_features	4m 39s
build_all_platforms (ubuntu-latest)	2m 19s
build_all_platforms (macos-latest)	2m 22s
build_default_features (ubuntu-latest)	48s
build_all_platforms (windows-latest)	5m 12s
build_default_features (macos-latest)	37s
build_default_features (windows-latest)	1m 21s
build_under_wasm	45s
check_msrv	32s
unit	6m 10s
	30m 55s

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