Alias improvement

[Valentin Tolmer]

The current structure of the symbol aliases is puzzling: why have the 
information duplicated only to resolve it later on?
I'm working on a patch to address this issue: the symbol and the alias 
will share a pointer to a separate structure containing the info. The 
symbol itself will have only its intrinsic info such as the name or 
location.

You can have a look at the patch here: 
https://github.com/nitnelave/bison/tree/symbol
I have yet to add some tests for what I added, but it is near completion
Valentin Tolmer