## Changes
While working on #1273, I found that calls to `Append` on a
`dyn.Pattern` were mutating the original slice. This is expected because
appending to a slice will mutate in place if the capacity of the
original slice is large enough. This change updates the `Append` call on
the `dyn.Path` as well to return a newly allocated slice to avoid
inadvertently mutating the originals.
We have existing call sites in the `dyn` package that mutate a
`dyn.Path` (e.g. walk or visit) and these are modified to continue to do
this with a direct call to `append`. Callbacks that use the `dyn.Path`
argument outside of the callback need to make a copy to ensure it isn't
mutated (this is no different from existing semantics).
The `Join` function wasn't used and is removed as part of this change.
## Tests
Unit tests.
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