constant propagation in optimizer

[ozanh]

As const declarations were introduced to uGO language, constant values should be propagated to expressions they are used in until they are shadowed. Optimizer does not create symbol tables but simple scopes which may help to propagate until a redefinition is encountered. Normally, users tend to avoid shadowing variables so constant value can be propagated most of the time.
This feature requires changes in both optimizer and compiler, and it should not extend the compilation process.