bytestring: Implement bytestring_spn() and bytestring_cspn()

Add bytestring_spn() and bytestring_cspn() functions which, in analogy to
strspn() and strcspn() return the lengths of initial sub-bytestrings which
either contain only a given set of bytes, or anything except a given set
of bytes.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>