optimization: avoid MonadErrorOps allocations

[fwbrasil]

Problem

The MonadErrorOps implicit class isn't an AnyVal, which ends up producing object instantiation in certain scenarios where the JIT compiler isn't able to elide the allocation. This is likely not an oversight since AnyVals can't have by-name parameters, which the class uses.

Solution

Use an implicit conversion + AnyVal class instead to avoid allocation while still allowing the by-name parameter.

Notes

• This change is part of Reduce allocation rate tapir#3552.
• I wonder if the by-name input is really needed since this class is meant to be instantiated on each use and will immediately evaluate the parameter to call the MonadError implementation.
• I'm not sure if the change is binary-compatible across the different Scala versions but I believe this is what implicit class desugars to.
• The conversion from => F[A] to () => F[A] doesn't require an allocation, both have the same representation at runtime.

[adamw]

There's 2 mima failures, unfortunately:

[error] core: Failed binary compatibility check against com.softwaremill.sttp.shared:core_3:1.3.17! Found 2 potential problems
[error]  * static method MonadErrorOps(scala.Function0)sttp.monad.syntax#MonadErrorOps in class sttp.monad.syntax does not have a correspondent in current version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("sttp.monad.syntax.MonadErrorOps")
[error]  * method MonadErrorOps(scala.Function0)sttp.monad.syntax#MonadErrorOps in object sttp.monad.syntax does not have a correspondent in current version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("sttp.monad.syntax.MonadErrorOps")

[adamw]

The by-name parameter I think is significant for handleError, which delegates to def handleError[T](rt: => F[T])(h: PartialFunction[Throwable, F[T]]): F[T] in MonadError - there could be exceptions when evaluating the r itself, which we want to catch.

[fwbrasil]

oh, I thought it could desugar to the same bytecode but there's the class constructor. I'll close this PR for now. I'm planning to try Graal and it might be able to get rid of these allocations better than C2.