Rewrite the AST matchers for slightly more composability.
Furthermore, check that the `starts_with` and `ends_with`
functions return a `bool`.
There is one behavioral change, in that the methods of a class (and
transitive classes) are searched once for a matching
`starts_with`/`ends_with` function, picking the first it can find.
Previously, the matchers would try to find `starts_with`, then
`startsWith`, and finally, `startswith`. Now, the first of the three
that
is encountered will be the matched method.
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Co-authored-by: Nicolas van Kempen <nvankemp@gmail.com>
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