Snapshot tests

Snapshot testing runs the compiler on many of the test projects in test/fixtures/execute and verifies that the compiled output is identical to a pre-calculated "snapshot". This verifies:

The compiler is deterministic (mostly)
The compiler's output does not change unexpectedly
Optimizations and type analysis in the generated code are working properly

These snapshots are automatically verified as part of the integration tests. There is also a more readable CLI for humans. To verify the snapshots:

node test/snapshot

When the compiler's output or the test projects have intentionally changed, run:

node test/snapshot --update

Verify that the snapshot diff is what you expect, then commit the changes.