Update comment.
Minor lattice code cleanup.
Fix bug in bootstrap blocks for lattice values.

If a bootstrap block installed a value into a lattice wrapper via "<=" AND no
other rules referenced that lattice wrapper, the value would never be installed
correctly, leading to an error in a subsequent tick about an orphaned delta
value.
Minor code cleanup.
Improve comment.
Improve the lattice rescan-on-merge and unsafe functions code.

This commit corrects two related unpleasantries. First, the code for detecting
calls to "unsafe" functions treated lattice identifiers as unsafe; hence, such
rules would be marked for reevaluation needlessly (e.g., even if there was no
delta to the associated lattice).

Second, correcting the first issue identified a shortcoming in the code for
rescanning collections upon lattice deltas. To reproduce the content of
collection "x", the previous code only marked the direct non-temporal
predecessors of "x" for rescan. In fact, we might also need to mark some
indirect predecessors for rescan. Right now, we only traverse back one
additional link in the dependency graph; on reflection, it might be necessary to
check all transitively reachable nodes, but I haven't seen a case where that is
warranted yet.
Fix #290.

When arranging to rescan collections upon deltas to a lattice value, we
neglected to consider two possibilities:

    (a) In a rule body that references a lattice, the LHS of the rule might also
        be a lattice; we assumed the rule LHS would be a traditional collection.

    (b) Similarly, we arranged for a rescan of all the "non-temporal" ancestor
        operators that feed the LHS collection. However, ancestor operators
        connected via the <+ or <~ operators might also need to be rescanned.
Merge branch 'master' of github.com:bloom-lang/bud