This PR implements lazy initialization of closed terms. Previous work
has already made sure that ~70% of the closed terms occurring in core
can be statically initialized from the binary. With this the remaining
ones are initialized lazily instead of at startup.
For this we implement a small statically initializable lock that goes
with each term. When trying to access the term we quickly check a flag
to say whether it has already been initialized. If not we take the lock
and initialize it, otherwise we dereference the pointer and fetch the
value.
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