Prefer "float" over "double"

[thekid]

function similarity(string $a, string $b): double {
  similar_text($a, $b, $p);
  return $p;
}

The function above yields TypeError (Return value of related\similarity() must be an instance of double, float returned, while (double)-casts work just as well as (float), and gettype(1.5) even returns double (one of PHP's inconsistencies...)

• Add lang.Primitive::$FLOAT, deprecate lang.Primitive::$DOUBLE
• Change casts inside code
• Change @param / @return and syntactic type hints
• Replace doubleval() with floatval()
• Replace is_double() with is_float()

[thekid]

The failing tests are not due to this refactoring but stem from use lang\Object being a (new) fatal error in PHP 7.2