Get element at compile-time index i.
Get element at index i.
Set element at index i to value.
http://forum.dlang.org/thread/gayfjaslyairnzrygbvh@forum.dlang.org#post-gayfjaslyairnzrygbvh:40forum.dlang.org
TODO: Merge with https://github.com/rcorre/enumap
TODO: Use std.range.indexed when I is an enum with non-contigious enumerators. Perhaps use among aswell.
TODO: Rename to something more concise such as Bby.
TODO: Allow I to be a string and if so derive Index to be that string.
Wrapper for R with Type-Safe I-Indexing.