Allow for files to be incrementally updated

[brendanzab]

Allow for files to be incrementally updated

[matklad]

One additional concern here is how to translate between BytePos -> LineCol coordinates in the presence of edits. In rust-analyzer, we hit this in the following case:

• our internal coordiantes are byte offsets
• the result of the refactoring is a set of edits (in byte-offset coordinates) and a final position of a curosr (byte offset in a file with changes)
• on the LSP layer, we need to translate this to (line, col) coordinates. Doing this for cursor offset is tricky.

This PR implements that logic: rust-lang/rust-analyzer#325

Additional random tidbit:

I was surprised that IntelliJ uses sorted array of offsets and not some fancy tree data structure, a-la xi-rope's monoid tree, to translate offsets: https://github.com/JetBrains/intellij-community/blob/513e5d6cdaa66c93e130393045bcc038537454d3/platform/core-impl/src/com/intellij/openapi/editor/impl/LineSet.java

In particular, they just update the array on changes: https://github.com/JetBrains/intellij-community/blob/513e5d6cdaa66c93e130393045bcc038537454d3/platform/core-impl/src/com/intellij/openapi/editor/impl/LineSet.java#L70-L94

IIUC, for things like grapheme clusters IntelliJ doesn't use indices at all, and instead does a linear scan withing a single line.

[brendanzab]

Thanks for the added information, that's really helpful!

[brendanzab]

Gonna close this because now user's can implement their own file databases using the Files trait.