hexdump: be a little more lock-friendly

hexdump: be a little more lock-friendly

hexdump processes a vector as a contiguous sequence of bytes, regardless
of V's value type, so hexdump should get a pointer and use uint8_t to
read the data.

Some vector types have a lock and some atomics in their operator[], so
let's avoid hammering those.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>