More precise mismatch error messages for polymorphic method invocations

[lrytz]

When invoking an effect-polymorphic method, the resulting effect consists of the latent, absolute effect, plus the effect of the argument.

The error message currently does not distinguish where the effect mismatch originates, so it is hard to find the cause of an effect mismatch sometimes:

  def h(f: Int => Int): Int @rel(f) = f(10)
    def test: Int @pure = {
    val x = 1
    h(x => {
      println()
      x + 1
    })
  }

The error is reported at the invocation of h, not at the println() statement:

T.scala:45: error: effect type mismatch;
 found   : @mod(any) @assign(any) @loc(any)
 required: @mod(x,f$1) @assign(x,any) @assign(f$1,any) @loc(any)
@mod(any) does not conform to @mod(x,f$1)
@assign(any) does not conform to @assign(x,any) @assign(f$1,any)
    h(x => {
     ^