Inverse lwtreslib and error_monad dependency

part of !1872 (closed)

depends on !2018 (merged)

Context

• make Lwtreslib independent of the error monad
• the errormonad provides TzLwtreslib which is Lwtreslib instantiated to the error monad

Intended as one of the multiple steps in modernising the error monad. (See !1872 (closed) for some other steps.) In particular, follow work includes: releasing Lwtreslib, improving Lwtreslib's Monad interface, making error-monad only about errors (registration, etc.) and instantiation.