head: splice fast-path for -c

[oech3]

Closes #11491 Needs cat from main branch.

$ truncate -s 1PB huge
$ cat huge | target/debug/head -c 1P|pv>/dev/null
^C.5GiB 0:00:02 [38.7GiB/s]

File input is not supported which is most common usecase.
This is draft until #11458 was merged.