Use := to change type in arrays

[bvssvni]

Similar to #5, but designed for arrays.

When using := in arrays, it should not allocate a new slot if the index is outside the range, since this might lead to confusing bugs. However, it can be used to overwrite the type, for example replacing a number with a string.

a = [5]

a[0] = "hello" // ERROR: Expected assigning to text

a[0] := "hello" // OK