Add lookup{LT,LE,GT,GE}, find{Min,Max}

[mtolly]

Closes #52. I needed the lookupXX functions to port some GHCJS code, and then I needed findMin/findMax to implement lookupLT/GT so I figured I'd export those as well.

All the functions also have thorough tests. I am new to PureScript so do let me know if you want any style changes or things like that :)

[garyb]

Looks good to me 👍

@paf31?

[paf31]

Looks great, but can we save some duplication by reversing orders perhaps? I'm not fussed either way, really, since the tests pass.

👍 Thanks!

[mtolly]

Sorry this dropped off my radar for a bit. Possibly you could condense them, but I don't see how to do it both elegantly and efficiently. You'd have to flip the whole tree and also reverse the comparison operator (probably with the Down wrapper). That'd be too much overhead for my tastes. Or you could have a shared implementation take a Boolean to make every left/right decision, but I think that'd make the code a lot less readable. If there were more potential savings it might make sense but here it seems not worth it.

[mtolly]

While I've got this PR open, would you be open to another PR that adds an Internal/Unsafe/similarly-named module exporting the Map constructors? I've had to write some fairly specialized functions for efficient Map traversal (https://github.com/mtolly/onyxite-customs/blob/faf480b/player/src/OnyxMap.purs#L383-L421) that require access to the constructors, so currently I'm using a local modified version of the module and it would be nice to not have to (i.e. less code to manage + smaller compiled blob size).

[mtolly]

Is this ready to be merged?

[garyb]

Can you rebase it please?

[mtolly]

Done, and updated to PS 0.9.1.

[garyb]

Thanks very much!

[garyb]

And sorry it got left for so long.