Nightly 2022-08-12

avm2: Implement 'throw' opcode

We currently lack the ability to preserve the original
`Value<'gc>` in the error, so we're forced to stringify the error.

This means that only typeless 'catch' blocks will work properly -
however, they're the only kind of 'catch' block that we currently
implement. Implementing support for typed 'catch' blocks will naturally
allow us to preserve the original 'Value<'gc>' in the 'throw'
implementation, since we'll need to switch to a custom `Error<'gc>`
type.