Implement more world manipulation functions in world DSL

[byorgey]

We should add more functions such as:

• reflectV
• reflectH
• offset/translate

See original design discussion at #1320 (comment) .

[byorgey]

It occurs to me that IF it were straightforward to add lambdas (I'm really not sure about that), then it would be simpler to add a single primitive imap with type (int -> int -> a -> b) -> W a -> W b, with which one could implement all of these (and others besides).

[byorgey]

Actually, I just realized we can do without lambdas in this case! We can give imap the type

imap : W int -> W int -> W a -> W a

The first two arguments say, for each location, the x- and y-coordinates to look up to get the contents for that cell. For example, to implement horizontal reflection you could use imap (-x) y. To implement rotation by 90 degrees you could imap y (-x), and so on.