Was broken by commit 8a86460b

Closes #87

We will reuse these tags elsewhere.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

By declaring the variable-length array to have a nominal size of 1,
struct TagPoolSlot shrinks from 24 bytes to 16 bytes, because "ref"
and "item" now both fit in one machine word.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

When a reference counter is at its limit, don't allocate a new
TagPoolSlot - that would result in many TagPoolSlot instances with
ref==1.  This in turn would make the linked list very very large,
which means quadratic runtime for many operations.

Fixes deadlock.