emit IR to substitute a tuple element by index

    t := (42, 'a', 3.14)

    var t2 := t
    t2[2] = 2.718

    print(t[2]) # 3.14
    print('\n')
    print(t2[2]) # 2.718
use IRBuilder::CreateMemCpy directly
improve a display format for float value
add println() builtin function template