Shorten type param names

[leshow]

I picked the most obvious letters to me. Mapping operations; F for function, for anything that returns a bool; P for predicate, etc. I made sure to change the associated lifetimes to match up and fix any formatting changes. Let me know if there's anything you'd like to change.

#249

[cuviper]

I'm not fond of changing ITEM to I -- that evokes "iterator" to my mind, especially when we're dealing with a library of iterators here. I'd prefer plain T for items, as the idiomatic type placeholder in Rust.

[nikomatsakis]

I no longer recall why I used M for the "base" operations, but it doesn't seem terribly intuitive to me. I see that the standard library uses I, perhaps we should use that (after adopting @cuviper's suggestion to change ITEM to T)?

[nikomatsakis]

If we were using T for Item, I think I'd prefer U for MAPPED_ITEM. If we're going into single letter territory, might as well go there, I guess. Somewhat oddly (to my mind), the standard library seems to adopt B for this purpose, though.

[nikomatsakis]

OK, looking over the changes, I'm torn. I do think the existing names like like MAP_OP and REDUCE_OP are not great. But the single-letter names sure do feel cryptic, particularly when you have a whole bunch in sequence (I'd be inclined to publish a "guide to the letters" in the docs somewhere, but it feels like if you need a guide to your names, you could have picked better names). Currently though we seem to have a mix, which is also not a win. I think all things considered I'm inclined to adopt this branch (after we finish bike-shedding the particular choices), since I'm on the fence and it aligns better with the prevailing style, but I'm mildly tempted to try and find better "one word" names (e.g., FUNC instead of MAP_OP) as an alternative.

[leshow]

I think those are good suggestions, (ITEM -> T, MAPPED_ITEM -> U) I can move on those later today so we can see what it looks like.

There is a few other cases where I ended up with a 2 letter params.
IDENTITY -> ID
OP -> OP
C_ITEM -> CI
PAR_ITER -> PI (following the prev suggestion this could just be I)

For C_ITEM, it occupies the same position as MAPPED_ITEM, so I think that should also become U if we make the aforementioned change. I don't see the 2 letter params as a problem, but if you want to strictly pursue the 1 character generic name then those will have to find replacements.

About documentation on the param names, I'm not an expert Rust developer and this library is new to me so my opinion may be worthwhile here, having the name MAPPED_ITEM didn't help me, looking at the constraints in the where clause did though.

[nikomatsakis]

Looks pretty good to me. I'm inclined to merge.

[nikomatsakis]

@cuviper you want to give one more look, or are you satisfied?

[cuviper]

I'm happy with this.

[leshow]

Thank you!