start to document MIR borrow check

[nikomatsakis]

Make an effort to start documenting the MIR borrow checker. In this first pass, all I managed to do was to write about move-paths and how they work.

Also, promote the borrow checker to its own top-level section.

[Mark-Simulacrum]

Reads well from a semi-outsiders perspective; very nice high level explanation.

[mark-i-m]

Thanks @nikomatsakis!

LGTM :)

I left a few minor comments/nits below.

[mark-i-m]

Did you intend to write more here? Could you leave a TODO?

[nikomatsakis]

Not sure if I did or not.

[nikomatsakis]

I have to say, I also find our "long line" check pretty annoying.

[nikomatsakis]

Seems like it should be more like a max limit of 90 or 100 or something.

[nikomatsakis]

Or, at minimum, less annoying for me to check locally (there isn't a convenient command).

[nikomatsakis]

ok, passes locally anyway...

[mark-i-m]

@nikomatsakis

Or, at minimum, less annoying for me to check locally (there isn't a convenient command).

MAX_LINE_LENGTH=80 ./ci/check_line_lengths.sh src/**/*.md

I'm fine with making the line length longer.

[mark-i-m]

If you want to make it 100 in this PR to make travis pass, I think that would be ok too...

[mark-i-m]

@nikomatsakis I would like to merge #184, but this will require you to fix a bunch of links in this PR and in others. Would you prefer for me to wait?

[nikomatsakis]

@mark-i-m ok I bumped to 100 so that summary.md can be committed (and just so that things are less irritating)

As for the upgrade, I guess it could go in any order, but I'd like to land these PRs, yes. What is the new link format?

[mark-i-m]

New: Relative to the current file
Old: Absolute paths

For example, if you are editing appendix/glossary.md and you want to link to traits/lowering-rules.md:

New: ../traits/lowering-rules.md
Old: ./traits/lowering-rules.md

[nikomatsakis]

@mark-i-m ah, nice. that confuses me all the time.

[nikomatsakis]

well I'm game for either order. I would say this PR is ready to land.