Experiment: restrict allowed trees in annotations

[mbovel]

Non-basic trees in type annotations currently cause compiler crashes. This experiment introduces restrictions on the trees allowed in annotations, similar to those in Java (§9.7. Annotations).

[mbovel]

scala3/tests/neg-custom-args/captures/reaches2.scala

Lines 4 to 5 in 5455c5d

	def compose1[A, B, C](f: A => B, g: B => C): A ->{f, g} C =
	z => g(f(z))

In this test, Ident(f) has type Function1[A, A], and is therefore not considered valid with the current rules. Why doesn't f have a singleton type?

f also does not have a symbol.

[mbovel]

scala3/tests/pos-macros/i7519b.scala

Lines 11 to 12 in 5455c5d

	val value: Expr[Int] = '{ 42 }
	'{ new Quoted[T @Annot($value)] }

Another question is what to do with splices, such as in the example above. Type splices are not valid anyway, so it might be okay to also refuse splices in type annotations?