Allow PDB to impl Send.

[afranchuk]

This allows PDB to be used across await points in async blocks. It also enables Sync access by e.g. wrapping with a Mutex.

This results in just one API-breaking change, where the Source::view trait method must now return a boxed trait object which is Send + Sync. If this breakage is unacceptable, maybe these changes could be behind a feature?

[mstange]

@jan-auer Could you take a look at this? It's needed for mstange/pdb-addr2line#63

[afranchuk]

Bump @jan-auer, any chance of this getting in?

[mstange]

I'm worried that not all consumers can provide a Send object as the parameter to PDB::open.

It would be nice if we could find a way to make PDB send whenever S is Send.

[afranchuk]

I think I've seen ways of doing this with trait objects, though (if I am remembering correctly) they are understandably hacky/cumbersome. It would be easier if this didn't store a dyn Msf. There's only one implementation of that, so we could avoid the trait object fairly easily...

[afranchuk]

TBH as of these changes, Source must be able to provide Send + Sync SourceViews, so I'd be surprised if Source was not at least Send too.