[stdlib] Add lstrip() and rstrip() to StringRef

[fknfilewalker]

lstrip and rstrip for StringRef, and delegates String.rstrip and friends to this StringRef implementation.

[JoeLoser]

Looks good, do you mind resolving the merge conflicts please and then ping me and I can import this PR? Thanks!

[fknfilewalker]

@JoeLoser

[JoeLoser]

@JoeLoser

Thanks for the ping, I'll TAL when I'm by a laptop soon, likely later today.

[JoeLoser]

Want to add some explicit tests for StringRef with these functions?

[fknfilewalker]

Want to add some explicit tests for StringRef with these functions?

I could copy the tests from string over, or do you have something better in mind?

[JoeLoser]

Want to add some explicit tests for StringRef with these functions?

I could copy the tests from string over, or do you have something better in mind?

I don't have anything better in mind, but I think that's what we've been doing with other recent PRs (although not consistently from a quick glance). It adds minimally more test coverage. What do you think?

In general, with all of this work in pushing String methods down to StringRef, maybe we should move a lot of String methods onto StringSlice instead, and on String have it internally take itself as a slice and do the operation? This was mentioned by @lsh offline the other day.