VFS should use a rope rather than strings

[nrc]

No description provided.

[nrc]

rust-dev-tools/rls-vfs#18

[ngortheone]

@nrc
I word of caution. I use xi-rope in my project and while most of the things are great there are downsides too. As an example unbounded searches in a rope are not cheap. See more details here:
xi-editor/xi-editor#1192

It may be possible to mitigate some of those, but specifically multilne regex searches are tough:

@cmyr
Doing multi-line regex efficiently would require a regex engine with a cursor API, which I don't see happening soon. :(

I don't want sound discouraging. Do your research and perf-testing before you do the switch.