Begin imposing hard limits on closures (#97)

[dktapps]

This change:

• prohibits non-static closures with a non-thread-safe $this bound
• restores $this onto target threads if $this is a thread-safe object
• disallows the use of static variables inside copied closures
• disallows the use of use-by-reference

There are a bunch more changes that need to be done, but this is a good start.

[dktapps]

Appears I forgot to commit the unit tests associated with this change.

[dktapps]

Test failures in jit-tracing runs are not related to this PR