Add missing tests for building AST from number
Move System.Decimal to System.Rational

Also move Rip::Core::Decimal to Rip::Core::Rational
Rip::Core::Integer#initialize should assume a Ruby integer gets passed in
Interpret Rip::Nodes::Decimal to produce Rip::Core::Rational
Implement and test System.Rational.{+,-,*,/,%,to_string}
Implement and test System.Rational.==
Implement Rip::Core::Rational.from_native
Interpret decimals [TR #53] (GH #8)

FIXME Integers and rationals are not yet interoperable (see https://trello.com/c/nbU2xQOB)