Garbage-collect obscured variables

[julianhyde]

Each time you define a variable in the shell, it adds a new value to the environment. If it has the same name as an existing variable, the old variable is not accessible, but it is still in the environment (implemented as a linked-list) and cannot be garbage-collected by the JVM.

I suspect that there are cases, such as where you define the same variable thousands of time, which have memory leaks. We should fix those.

The linked-list environment is good in general, but we need to find a way to purge it, without paying an O(n) cost to check the whole environment every time.

[julianhyde]

Fixed in 297c3be.