Add FromResp impl for HashMap

[augustuswm]

This PR adds an implementation of FromResp for HashMap. It takes an array response containing an even number of elements and treats it as a list of alternating key, value elements.

KEY1
VALUE1
KEY2
VALUE2

This can be used to transform the response of HGETALL into a HashMap<K, V> where K and V implement FromResp.

Not sure if this is in scope for this library, but here it is in case others find it useful.

Ran in to this in an exercise to try porting an existing project from redis-rs to redis-async-rs. Heavily inspired by https://docs.rs/redis/0.9.1/src/redis/types.rs.html#844-860

[benashford]

Cool 😃

Thanks for this 👍

[benashford]

Published as 0.4.3

I made a small tweak which means it should work for HashMaps with other hashers, e.g. FnvHashMap from here: https://github.com/servo/rust-fnv but this is transparent to the calling code (i.e. the tests you added are unmodified and work the same way).

Thanks again.