Range adaptor: cache

As discussed in this thread:
http://forum.dlang.org/thread/ovbjcmogezbvsxrwfcol@forum.dlang.org

This provides a range adaptor that cache the result of another range.

Meant as a lazy alternative to array. Not much else to say...?

Implemented as obscure type

Wording in documentation might suck.

Please destroy.
Instantiate*

I don't think referring to `Cache` in error messages (or documentation) is wise as long as `Cache` is undocumented.