Allow terminal return as result of block expression

[som-snytt]

One solution to organizing local functions is to allow a terminal return expression in a prefix of statements in a block where the suffix are constrained (perhaps to be defs only, perhaps the next expression must be the inferred unit); then the returned expression is allowed to guide inference of the result type of the enclosing method.

The motivation is to make the result (here, in the return expression) easier to locate than if it were at the bottom of the block.

def f = {
  return g
  def g = 42
}

Another proposal to distinguish local defs from other statements is lampepfl/dotty-feature-requests#231