Feature/multieffects

[b-studios]

Implement multi effects.

For now we decided on the following syntax.

Effect declarations

effect State {
  def get(): Int
  def set(n: Int): Unit
}

Effect calls

do get()

Effect handling

try { ... } with State {
  def get() = resume(4)
  def set(n: Int) = ...
}

Shorthands

There are also shorthands for singleton operations

effect Foo(): Unit

currently expands to

effect Foo {
  def Foo(): Unit
}

and

try { ... } with Foo { () => ... }

expands to

try { ... } with Foo { 
  def Foo() => ... 
}

[b-studios]

Mentioning the effect type on handling has the advantage that later, we can bind type parameters there.

[b-studios]

Great! Thanks for updating the examples and tests!