[Lang] Explicit new variable and hide variable

[windwhiterain]

In many language e.g. rust, we can use let a = 1 to create a new variable. However, in taichi set the value of a variable and create a new variable writes in the same way. I made two hints: taichi.new and taichi.hide that used in annoted assign to specify if we want to create a new variable in current scope or hide a variable in previous scope with a new variable.

[feisuzhu]

/rebase