Add ability to store complex number in the variable

[sys27]

Example: x := 5+2i

Related to #131

[sys27]

Right now, values can be saved into two collections: variables and functions. The variables collection is a 'static' value, which is computed on assign. The functions collection is a 'dynamic' value, which will be computed on execution of expression.
So, functions can store any value. Do we really need variable collection? Possible, we have to use only function collection and use functions as first class objects like in Functional Programming, can affect performance. Or just merge these two entities to single collection of objects?