memory optimization: use union to store value for different types

[mw66]

ChickenClisp/source/orelang/Value.d

Line 39 in 815c874

private {

This consumes too much memory, it's better to use union to store value for different types.

Each Lisp cell can only store one type information, you are wasting too much memory.

[mw66]

Or do it in D's OOP way, by define class hierarchy:

https://github.com/zephyrfalcon/fairy-lisp/blob/master/source/types.d#L119