Hide valkeyContext

Hi,

I'd like to hide valkeyContext into libvalkey internal data structure with several changes:

- use `typedef void *valkeyContext;` instead in `valkey.h`, redefine `struct valkeyContext` in `valkey_private.h`
- use `valkeyContextGetErr(valkeyContext c)` instead of `c->err`, also apply this rule to `c->errstr`

Does anything block this?