Rise's capture avoiding substitution is bugged

[Bastacyclop]

In #145 I found out that our capture-avoiding substitution was bugged. The condition to alpha-rename should be FV(expression) contains x and not FV(b) contains x:

shine/src/main/scala/rise/core/substitute.scala

Lines 33 to 43 in 9b04bf2

	case Lambda(x, b) =>
	// See https://www.cs.cornell.edu/courses/cs3110/2019sp/textbook/interp/subst_lambda.html
	if (x =~= `for`) return_(e)
	// FIXME: should be !(FV(expression) contains x)
	if (!(FV(b) contains x))
	super.expr(e)
	else {
	val newX = Identifier(freshName(x.name.substring(0, 1)))(x.t)
	val newB = replace.exprInExpr(newX, `for`=x, in=b)
	super.expr(Lambda(newX, newB)(e.t))
	}

However changing the alpha-renaming condition breaks tests like harrisCornerDetectionHalideCheck and cameraPipelineCheck, indicating another bug in our codebase. My intuition tells me that it could be related to #81, since alpha-renaming more often reduces the risk of having non-unique names.
@michel-steuwer @umazalakain