Feat: support for expression in computed member expressions.

[pacifiquem]

Now we support expressions in computed member expressions!:

reka obj = {
   key: "value",
   key2: "value2"
}

reka value = obj[injiza_amakuru(" enter key's name you want the value of: ")]  # if input is key2
tangaza_amakuru(value) # This will print value2

Any expressions can now work for computed value.