Revert "Make left zero dynamic (rewrite)"

[gusty]

Reverts #503 because of this:

> open FSharpPlus.Control ;;
> IsLeftZero.Invoke empty<OptionT<Task<Option<int>>>> ;;
Binding session to 'C:/Users/gmpl2/.nuget/packages/fsharpplus/1.3.0-ci02744/lib/net6.0/FSharpPlus.dll'...
System.TypeInitializationException: The type initializer for 'FSharpPlus.Control.IsLeftZeroHelper`1' threw an exception.
 ---> System.ArgumentException: The type or method has 2 generic parameter(s), but 1 generic argument(s) were provided. A generic argument must be provided for each generic parameter.
   at System.RuntimeType.SanityCheckGenericArguments(RuntimeType[] genericArguments, RuntimeType[] genericParameters)
   at System.Reflection.RuntimeMethodInfo.MakeGenericMethod(Type[] methodInstantiation)
   at <StartupCode$FSharpPlus>.$Applicative..cctor$cont@195-1[a](Type ty, Type[] targs, Unit unitVar)
   at <StartupCode$FSharpPlus>.$Applicative..cctor$cont@172[a](Type ty, Type helperTy, Type[] targs, Unit unitVar)
   at FSharpPlus.Control.IsLeftZeroHelper`1..cctor()
   --- End of inner exception stack trace ---
   at FSharpPlus.Control.IsLeftZeroHelper`1.Invoke(a x)
   at <StartupCode$FSI_0006>.$FSI_0006.main@() in C:\Lula\stdin:line 5
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
Stopped due to error

that's because Empty on OptionT (as well as in other monads) uses SRTPs

therefore stuff like

> sequence [OptionT [Some 1]; OptionT [None]] ;;
System.TypeInitializationException: The type initializer for 'FSharpPlus.Control.IsLeftZeroHelper`1' threw an exception.
 ---> System.ArgumentException: The type or method has 2 generic parameter(s), but 1 generic argument(s) were provided. A generic argument must be provided for each generic parameter.
   at System.RuntimeType.SanityCheckGenericArguments(RuntimeType[] genericArguments, RuntimeType[] genericParameters)
   at System.Reflection.RuntimeMethodInfo.MakeGenericMethod(Type[] methodInstantiation)
   at <StartupCode$FSharpPlus>.$Applicative..cctor$cont@195-1[a](Type ty, Type[] targs, Unit unitVar)
>    at <StartupCode$FSharpPlus>.$Applicative..cctor$cont@172[a](Type ty, Type helperTy, Type[] targs, Unit unitVar)
   at FSharpPlus.Control.IsLeftZeroHelper`1..cctor()
   --- End of inner exception stack trace ---
   at FSharpPlus.Control.IsLeftZeroHelper`1.Invoke(a x)
   at <StartupCode$FSI_0007>.$FSI_0007.main@() in C:\Lula\stdin:line 6
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)
Stopped due to error

won't work

Its really a shame but I think we can keep these bits and use them in other places where this situation doesn't happen.