There are two issues with the implementation of `provideCodeActions`
introduced in #1439:
1. We're returning the code action based on the file its diagnostic is
in; not the file the suggested fix is in. I'm not sure how often
fixes are suggested cross-file but it's something we should handle.
2. We're not filtering code actions based on the passed range. The means
if there is any suggestion in a file we'll show an action for every
line of the file. I naively thought that VS Code would filter for us
but that was wrong.
Unfortunately the VS Code `CodeAction` object is very complex - it can
handle edits across multiple files, run commands, etc. This makes it
complex to check them for equality or see if any of their edits
intersects with a specified range.
To make it easier to work with suggestions this introduces a
`SuggestedFix` model object and a `SuggestFixCollection` code action
provider. This is a layer between the raw Rust JSON and VS Code's
`CodeAction`s. I was reluctant to introduce another layer of abstraction
here but my attempt to work directly with VS Code's model objects was
worse.
abc0784e57