Allow queries to be eagerly constructed

[kyri-petrou]

An issue with ZQuery that makes it hard to optimize in libraries like Caliban is that it doesn't provide any means to eagerly construct queries who's values can be extracted without executing the queries via the ZIO runtime.

Let's take this as an example:

val allCountryNames = Set(???)
val datasource = ??? // DB

def amIAllowedToDrive(countryName: String, age: Int): ZQuery[Any, Throwable, Boolean] = {
  if (age < 16) ZQuery.succeed(false)
  else if (!allCountryNames.contains(countryName)) ZQuery.fail(new Throwable("invalid country name"))
  else datasource.query(???)
}

In the example above, the first 2 branches do not require us to run the ZQuery in order to evaluate it as the outcome is already known.

This PR exposes xNow variants of different methods that allow users to eagerly construct ZQueries. Libraries (like Caliban) can then check if a ZQuery is eagerly constructed via the asExitMaybe and asExitOrElse methods and extract the value/failure from it without the need of running the ZQuery and providing a Cache to it.

[paulpdaniels]

Are we ok with having this part of the public API? I know historically the core has made *Now stuff package private to avoid footguns.

[kyri-petrou]

Personally I think it's okay. Most (if not all) other frameworks, including Scala's Future (which I know isn't the best example because it's not referentially transparent) expose methods for eagerly constructing monads. I think in ZIO having the *Now methods makes less sense because it has Exit. In ZQuery users don't really have that option.