surf: Rename `Repository::blob` to `blob_at` and add new `blob` fn #719

Job	Run time
build-windows	3m 12s
build-linux (stable)	54s
build-linux (nightly)	52s
build-macos	1m 13s
cargo-deny (advisories)	21s
docs	41s
cargo-deny (bans licenses sources)	19s
lint	43s
fmt	17s
test-linux	0s
test-macos	0s
	8m 32s

Provide feedback