Get rid of `Monadic.plist` and `Monadic.pstring`

They're not generic enough, and can be easily implemented at consumer
side, e.g.

```ocaml
let plist = fmap Int64.to_int uint64 >>= list t
let pstring = uint8 >>= bytes
```
Add `Monadic.array`

Pretty sure this could be implemented as part of `Applicative` though...
Minor fixup