fix: SourceView implement Sync

[underfin]

The *const u8 is !Send, here already using Mutex to make sure the ptr is thread-safe, so add SourceViewLinePtr struct to let it implement Send + Sync.

[Swatinem]

The reason this is a (*const u8, usize) in the first place is I believe a historic artifact.

In reality it should be &'a str, as it points into the source. One way or another, this can only be done using unsafe code, as you can potentially create self-referential data that way (because pointing to a Cow::Owned makes it self-referential).

I would rather fix this in such a way that we only have a single place for the unsafe code, namely where we create these &'a str slices.

All the other unsafe code, like using raw pointers, the make_str fn, etc should rather be removed.

SourceView pretty much is just a caching, on-demand version of .lines().collect().

[underfin]

@Swatinem Thanks for your review, I already fixed it.

[Swatinem]

I’m surprised this needs changes to the DecodedMap

[underfin]

@Swatinem Could you publish a new version for it? Thank you a lot.

[Swatinem]

Done. Though I somehow messed up the commit message when merging, on well.