RationalNode numerator & denominator fields to make it directly usable

[eregon]

Rational literals are either:

• 123r
• 12.34r

(12ri/12.34ri is just ImaginaryNode of RationalNode, so a trivial variant)

These are actually quite different, the first is the same as Rational(123, 1) but the second is Rational(1234, 100), not Rational(12.34):

irb(main):001:0> 12.34r
=> (617/50)
irb(main):002:0> Rational(1234, 100)
=> (617/50)
irb(main):003:0> Rational(12.34)
=> (3473401212609495/281474976710656)

So right now to handle this we have to manually parse the number, e.g. TruffleRuby does it like this: https://github.com/oracle/truffleruby/blob/5096dddf6c3adc7edbae0938efc4fb4913b62bba/src/main/java/org/truffleruby/parser/YARPTranslator.java#L2905-L2927

It would be great if RationalNode was changed to have a numerator and denominator field, then we wouldn't need to manually reparse the number. It could also potentially save an unnecessary conversion from string to double, since the double value in this case should not be used anyway.

code	before	after
123r	RationalNode(numeric=IntegerNode(123))	RationalNode(numerator=IntegerNode(123), denominator=IntegerNode(1) (or nil))
12.34r	RationalNode(numeric=FloatNode(12.34))	RationalNode(numerator=IntegerNode(1234), denominator=IntegerNode(100))

cc @andrykonchin