verityutils: s/vector/span/

while writing I8fe9dc047af729b0f5730ec1844b3902225fa73e, i noticed that
we're making a temp vector just to call toHex.

since toHex only iterates over and inspects its argument, it can take
a `span` instead, which allows us to avoid making a copy.

Bug: None
Test: TreeHugger
Change-Id: Ide87b7ab7a10e68e895338fb61a41af6c1ab6e77
1 file changed