* Use `TryFlags` instead of hard-coded binary literals.
* Rename `try_length`, `catch_length`, `finally_length` to `try_size`,
`catch_size`, `finally_size` to match SWF19 namings.
* Refactor write and fix a few bugs there:
* The actions length should not include the try, catch, finally
bodies, only the metadata of `flags`, `try_size`, `catch_size`,
`finally_size` and catch variable (either as `u8` or `SwfStr`).
* A placeholder byte should be written in place of the catch variable
when there is no catch clause.
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