Allow block before lambda_expression

[ptwales]

This is sort of not conventional but is valid:

val foo: Int => Int = { x => 
  val y = 2 * x
  y * y
}

Output from playground:

val_definition [0, 0] - [3, 1]
  pattern: identifier [0, 4] - [0, 7]
  type: function_type [0, 9] - [0, 19]
    parameter_types: parameter_types [0, 9] - [0, 12]
      type_identifier [0, 9] - [0, 12]
    return_type: type_identifier [0, 16] - [0, 19]
  value: block [0, 22] - [3, 1]
    lambda_expression [0, 24] - [1, 15]
      identifier [0, 24] - [0, 25]
      assignment_expression [1, 2] - [1, 15]
        left: postfix_expression [1, 2] - [1, 7]
          identifier [1, 2] - [1, 5]
          identifier [1, 6] - [1, 7]
        right: infix_expression [1, 10] - [1, 15]
          left: integer_literal [1, 10] - [1, 11]
          operator: operator_identifier [1, 12] - [1, 13]
          right: identifier [1, 14] - [1, 15]
    infix_expression [2, 2] - [2, 7]
      left: identifier [2, 2] - [2, 3]
      operator: operator_identifier [2, 4] - [2, 5]
      right: identifier [2, 6] - [2, 7]

I don't know what this should parse to but val y should be a val_definition and not a post_fix_expression

[ptwales]

for the record this is also valid and the same:

val foo: Int => Int = { 
  x => val y = 2 * x
  y * y
}

[susliko]

Resolved by #277!