0.2.7

This release includes a ground-up rewrite of the regex-syntax crate, which has
been in development for over a year.

New features:

• Error messages for invalid regexes have been greatly improved. You get these
  automatically; you don't need to do anything. In addition to better
  formatting, error messages will now explicitly call out the use of look
  around. When regex 1.0 is released, this will happen for backreferences as
  well.
• Full support for intersection, difference and symmetric difference of
  character classes. These can be used via the &&, -- and ~~ binary
  operators within classes.
• A Unicode Level 1 conformat implementation of \p{..} character classes.
  Things like \p{scx:Hira}, \p{age:3.2} or \p{Changes_When_Casefolded}
  now work. All property name and value aliases are supported, and properties
  are selected via loose matching. e.g., \p{Greek} is the same as
  \p{G r E e K}.
• A new UNICODE.md document has been added to this repository that
  exhaustively documents support for UTS#18.
• Empty sub-expressions are now permitted in most places. That is, ()+ is
  now a valid regex.
• Almost everything in regex-syntax now uses constant stack space, even when
  performing anaylsis that requires structural induction. This reduces the risk
  of a user provided regular expression causing a stack overflow.
• FEATURE #174:
  The Ast type in regex-syntax now contains span information.
• FEATURE #424:
  Support \u, \u{...}, \U and \U{...} syntax for specifying code points
  in a regular expression.
• FEATURE #449:
  Add a Replace::by_ref adapter for use of a replacer without consuming it.

Bug fixes:

• BUG #446:
  We re-enable the Boyer-Moore literal matcher.