Allow operators as method names

[faultyserver]

The parser and native library already implement +, -, *, / as Calls to methods on objects. This should also be allowed in definitions within the language. For example:

deftype Foo
  def a; @a; end

  def initialize(a)
    @a = a
  end

  def +(other : Foo)
    @a = @a + other.a
  end
end

f1 = %Foo{1}
f2 = %Foo{2}

f3 = f1 + f2
f3.a #=> 3

This is a contrived example, for sure, but shows how the syntax can be used. This is particularly helpful for things like DateTimes or numeric replacements (e.g., BigInteger).

The only change needed for this should be allowing the operators as method names. The parser should already handle the infix calls properly.

The operators that should be supported are: +, -, *, /, %, <, <=, ==, !=, >=, >, [], []=, and =: (see #11). Handling unary operators (-, !, *) can come later, since the interpreter doesn't currently support these anyway.