Refactor core::ops, move RangeArgument to libcore::ops

[clarfonthey]

This is a redone version of most of #41460 that moves RangeArgument to core::ops. This also refactors core::ops into submodules so that it's a bit easier to tell where things are.

This relies on #42268.

[rust-highfive]

r? @aturon

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[Mark-Simulacrum]

Looks like travis failed:

[00:01:37] error: expected identifier, found keyword `fn`
[00:01:37] --> /checkout/src/libcore/ops/mod.rs:151:5
[00:01:37] |
[00:01:37] 151 | mod fn;
[00:01:37] | ^^
[00:01:37] [00:01:37] error: aborting due to previous error
[00:01:37] [00:01:37] error: Could not compile `core`.

[bors]

☔ The latest upstream changes (presumably #42348) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

@clarcharr Looks like this is still waiting on some fixes from you... I guess still waiting on #42268 though.

[aidanhs]

r? @alexcrichton
Since you've been actively involved with the history here (#41460, #42268) I figure you may want to take a look at this one.

@clarcharr is it possible/does it make sense for you to separate out the two PRs? I realise it's probably a bit of a pain to unpick, but it looks like the other one may be stalled for now?

[clarfonthey]

@aidanhs I will do that! it makes sense either way

[clarfonthey]

Also I have to redo this anyway because of the Carrier -> Try move.