Simpler server trait for infallible operations

[mkeeter]

Infallible operations (i.e. those using Simple(T) or Result<T, ServerDeath> modes) currently require a signature

Result<T, idol_runtime::RequestError<core::convert::Infallible>>

in the server-side trait.

We should simplify this to just T to make writing servers more ergonomic. This would require minor Hubris changes, as there are only two Idol APIs using Simple right now.

[cbiffle]

We can simplify this to T in some cases, but not all -- RequestError is used for faulting clients that send bogus arguments, etc.