Nightly 2021-10-03

swf: Improve `ActionTry` read/write

* 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.