Add support of rational number to trigonometric functions

[sys27]

Currently, the rational number is implemented in the following way: it has two NumberValues (numerator and denominator) and can represent only "numerical" rational values, like: 1 // 3. It provides basic abilities to work with rational numbers (+, -, *, /, ^, log) while keeping higher precision compared with irrational numbers, 1 // 3 vs 0.33333....

I think it could be helpful in trigonometric/hyperbolic functions:

sin(2pi // 3) = sqrt(3) // 2

So, the user can use variables and other functions inside rational numbers. For example, instead of using 2.0943951023931953 radians in the sine function, you will be able to write 2pi // 3 and it will be evaluated into the rational number sqrt(3) // 2 instead of 0.8660254037844386.

Questions:

• How to represent the rational number? Should RationalValue store two references to IExpression? Or should we use the Rational class directly and remove RationalValue?
• Do we need to add symbolic evaluation, like 3 * (sqrt(2) // 2) -> 3*sqrt(2) // 2?
• If so, when do we need to use symbolic evaluation and when numeric (2 * (1 // 3) -> 2 // 3)?
• Will it be helpful for #39?