compiler.tree.modular-arithmetic: more use of hash-sets.
tools.crossref: speed up build-crossref.
destructors: use hash-sets to keep track of disposables.
prettyprint.sections: more use of hash-sets.
regexp.dfa: more use of hash-sets.
tools.completion: Add utility word for searching for a string in a sequence of strings.
combinators.extras: Add twice, thrice words.
hash-sets: Support >alist on hash-sets or else tab-completion breaks. Can we write a unit test for this too?
ui.tools.listener.completion: fix use of search-vocab-names.
hash-sets: back out this change for now.
graphs: change closure to use a set and operate on sequences.
Revert "graphs: change closure to use a set and operate on sequences."

This reverts commit 2cc201a38ac6110f0b8c9c86f4cb34c1fde43c0f.
tools.crossref: fix help-lint.
sets: implement M\ f delete.