Add Ratio type

[GreasySlug]

Fixes #218.

Changes proposed in this PR:

• Add Ratio type

  • Add ratio.rs
    • add
    • sub
    • div
    • rem
    • mul
    • eq
    • PartialEq
  • Add _erg_ratio.py
    • Ratio class
    • RatioMut class
    • Ratio.d.er

• TODO

  • integration tests
  • declare of Float type

Removed

These contents are excluded

• Add Complex type
  • Implement complex type by using ratio.rs
  • Add _erg_complex.py
  • Add ComplexLit, Im token
  • parse Im token
  • convert Im token and num calc to ComplexLit
• Improve Int, Nat, Bool type

Currently known bugs

• import "consts"
  infinite loop? or so long runtime
• tests/should_ok/structural.er
• mut_array.er
• Cannot define Float type
• print! 0.3
  runtime is 3/10 but test code eval is 0.3

---

@mtshiba

[mtshiba]

The display of rational numbers is not consistent. It would be better to display them in the form numerator/denominator (e.g. 3/10).

>>> 3/10
Ratio(3, 10)
>>> 0.3
(3, 10)
>>> 2.3 % 2
Fraction(3, 10)