Add `--keep-symbols` flag (#302)

* Add `--keep-symbols` flag

* Replace pwasm_utils::optimize by a simple export stripper

* Satisfy clippy

* Fix typos

Co-authored-by: Michael Müller <[email protected]>

* Fix test build errors

* Fix tests

* Rename to `--keep-debug-symbols`

* Add test for `--keep-debug-symbols`

* Fix typos

Co-authored-by: Andrew Jones <[email protected]>
Co-authored-by: Michael Müller <[email protected]>

* Restore when/then

Co-authored-by: Michael Müller <[email protected]>
Co-authored-by: Andrew Jones <[email protected]>