Try to fix capture flow

[b-studios]

Fixes #362 and #365 which surprisingly were the result of the same problem.

effekt/effekt/shared/src/main/scala/effekt/Typer.scala

Lines 1092 to 1097 in e0d28ef

	(bps zip bargs zip captArgs) foreach { case ((tpe, expr), capt) =>
	usingCapture(capt)
	given Captures = capt
	val Result(t, eff) = checkExprAsBlock(expr, Some(tpe))
	effs = effs ++ eff
	}

Strangely the given instance provided after usingCapture still shadows the previous call.

Introduced new combinator flowingInto to make scoping more explicit.

effekt/effekt/shared/src/main/scala/effekt/Typer.scala

Line 1192 in 69526c2

def flowingInto[T](c: Captures)(prog: Captures ?=> T): T = prog(using c)

Now the above code is

effekt/effekt/shared/src/main/scala/effekt/Typer.scala

Lines 1095 to 1103 in 69526c2

	(bps zip bargs zip captArgs) foreach { case ((tpe, expr), capt) =>
	flowsInto(capt, callsite)
	// capture of block <: ?C
	flowingInto(capt) {
	val Result(t, eff) = checkExprAsBlock(expr, Some(tpe))
	effs = effs ++ eff
	}
	}

which explicitly scopes the implicit Captures context.

All other changes are clean-up / refactoring changes.

IMO this is a problem with Scala, reproduced here: https://scastie.scala-lang.org/dNPmyHdGSdKJ0atotRofPw