Store Request::params as a slice instead of owning ArrayVec (sere…

…nity-rs#2694)

This,
1. shrinks the size of Request, when copied around, as it doesn't have
to store the max capacity at all times
2. shrinks llvm-lines (compile time metric) for my bot in debug from
`1,153,519` to `1,131,480` as no monomorphisation has to be performed
for `MAX_PARAMS`.