struct environment

[Andrew Cagney]

> 
> Why do you want to have multiple available implementations?  I think
> the overhead for always hashing is small enough.  There are only two
> reasons struct block allows hash tables or linked lists:
>   - overloading of the meaning of that list to represent a list of
>     function arguments, which is ordered
>   - warts in mdebugread that I was not patient enough to overcome when
>     I finally merged in hash table support
> 
> I suppose the first reason is a legitimate one for multiple
> implementations; we could mark an environment as 'ordered'.  Or we
> could stop overloading the meaning of the list that way.  I don't know
> which is better.

Daniel's observation is correct.  The last thing we need is something 
totally overengineered.  Is there any reason to not start out with a 
very simple implementation and get it working first?

Only once the structure is working correctly should we be thinking about 
making it work fast.

Andrew