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The principle change in this commit is a complete rewrite of how literals are detected from a regular expression. In particular, we now traverse the abstract syntax to discover literals instead of the compiled byte code. This permits more tuneable control over which and how many literals are extracted, and is now exposed in the `regex-syntax` crate so that others can benefit from it. Other changes in this commit: * The Boyer-Moore algorithm was rewritten to use my own concoction based on frequency analysis. We end up regressing on a couple benchmarks slightly because of this, but gain in some others and in general should be faster in a broader number of cases. (Principally because we try to run `memchr` on the rarest byte in a literal.) This should also greatly improve handling of non-Western text. * A "reverse suffix" literal optimization was added. That is, if suffix literals exist but no prefix literals exist, then we can quickly scan for suffix matches and then run the DFA in reverse to find matches. (I'm not aware of any other regex engine that does this.) * The mutex-based pool has been replaced with a spinlock-based pool (from the new `mempool` crate). This reduces some amount of constant overhead and improves several benchmarks that either search short haystacks or find many matches in long haystacks. * Search parameters have been refactored. * RegexSet can now contain 0 or more regular expressions (previously, it could only contain 2 or more). The InvalidSet error variant is now deprecated. * A bug in computing start states was fixed. Namely, the DFA assumed the start states was always the first instruction, which is trivially wrong for an expression like `^☃$`. This bug persisted because it typically occurred when a literal optimization would otherwise run. * A new CLI tool, regex-debug, has been added as a non-published sub-crate. The CLI tool can answer various facts about regular expressions, such as printing its AST, its compiled byte code or its detected literals. Closes #96, #188, #189