goplus 0.9.7 rational number support issue

[qiukeren]

the playground sample is on main branch, errors can be reproduced on 0.9.7.

playground 测试代码使用main分支，问题都能使用0.9.7复现。

1. Cannot direct print a rational number 3.14r, directly print 3r is ok.

无法直接打印3.14r，可以直接打印3r

https://goplay.qiniu.com/p/valNl8Kgmoe

println(3.14r)

it panics directly.

print(3r)

2. Rational number cannot multiply a integer.

无法和整数直接相乘。

https://goplay.qiniu.com/p/vXcwZIYlkqT

x:=3r
x*=2
println(x)

it can not convert the type.

3. Cannot use the *= operator

问题比较类似2，无法使用*=之类的运算符来计算，也是因为类型转换的问题。

https://goplay.qiniu.com/p/4l3GbhMZtfF

x:=3r
x*=2r
println(x)

[xushiwei]

57befb4