Resume compilation after exception

[denisdefreyne]

Support incremental compilation by allowing compilation to resume after an exception

Detailed description

Before:

1. Load checksums, dependencies, …
2. Determine outdatedness
3. Compile all items
4. Write new checksums, dependencies, …

After:

1. Load checksums, dependencies, …
2. Determine outdatedness
3. Remember all currently outdated items in a new outdatedness store
4. Write new checksums, dependencies, …
5. Compile all items; after each item, compilation mark the compiled item as no longer outdated
6. Write new outdatedness store

The outdatedness checker will not be integrated with the outdatedness store.

To do

• Do not store non-existant items in OutdatednessStore
• Create OutdatednessStore in CompilerLoader

Issues

• The OutdatednessStore is stored twice. This isn’t really a problem, but there’s no clear/clean way to fix that right now.