SI-6308 Specialize methods that have some unspecialized params.

This is a continuation of 1591c14e50, which didn't go far enough
to handle method calls with a mix of specialized and unspecialized
type parameters.

This commit modifes `specSym`

    tree              = Main.this.f4[Nothing, Int]
    tree.tpe          = (a: Int, b: List[(Int, Nothing)])String
    fun.tpe           = [B, A](a: A, b: List[(A, B)])String
    residualTreeType  = [B](a: Int, b: List[(Int, B)])String
    memberType        = [B](a: Int, b: List[(Int, B)])String
    env               = Map(type A -> Int)
    doesConform       = true