Fix ExceptionPackage memory errors (#3088)

First, adds an explicit destructor call to fix a memory leak in
`Literal::operator=` in which existing `ExceptionPackage`s would be
silently dropped. Next, changes `Literal::getExceptionPackage` to
return the `ExceptionPackage` by value to avoid a use-after-free bug
in the interpreter that was surfaced by the new destructor call.

A future improvement would be to switch to using `std::variant`.

Fixes #3087.

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