Clean up as_byte_slice vs as_bytes (etc)

[Manishearth]

The zerovec crate uses byte_slice and bytes to talk about methods working with &[u8]. We should pick one (I suggest as_bytes() etc since that's what the stdlib does)

$ rg "\b[a-zA-Z_]*_byte_slice[a-zA-Z_]*\b" --no-filename --only-matching -N | sort -u
as_byte_slice
from_byte_slice_unchecked
from_byte_slice_unchecked_mut
impl_byte_slice_signed_type
impl_byte_slice_size
impl_byte_slice_type
impl_byte_slice_unsigned_type
iule_from_byte_slice_unchecked_mut
parse_byte_slice
parse_byte_slice_with_length
parse_owned_byte_slice
validate_byte_slice

*_bytes* usage

$ rg "\b[a-zA-Z_]*_bytes[a-zA-Z_]*\b" --no-filename --only-matching -N | sort -u
as_bytes
as_encoded_bytes
bincode_bytes
_bytes
char_bytes
deserialize_bytes
from_bytes
from_bytes_unchecked
from_bytes_unchecked_mut
from_bytes_unchecked_with_length
from_le_bytes
get_bytes_at_mut
get_serializable_bytes_non_empty
indices_bytes
into_bytes
large_hashmap_postcard_bytes
large_zerohashmap_postcard_bytes
large_zeromap_postcard_bytes
last_field_bytes
le_bytes
len_bytes
postcard_bytes
serialize_bytes
shift_bytes
to_be_bytes
to_le_bytes
try_from_bytes
ule_bytes
unsized_bytes
visit_borrowed_bytes
vzv_from_bytes
write_serializable_bytes
write_serializable_bytes_without_length
zv_bytes

Everything but the impl ones should probably be changed,