Make intptr_union an std::variant

[Nydauron]

Currently, we've modified intptr_union to be the following definition:

typedef struct {
    int i;
    void *p;
    std::shared_ptr<char> s;
} intptr_union;

before that change, it was:

typedef union {
    int i;
    void* p;
} intptr_union;

This change was made to help fix some memory leaks within the parser.

We ideally want to wrap all 3 different types in a std::variant since that behaves exactly like union but allows for objects to be constructed/destructed, but currently, there some Spim test files that are causing the parser to panic due to a mismatch of type assert. If we can, we should add type checking before we unwrap the std::variant and handle accordingly if it is a different value.

To see what it was like when we first tried to use std::variant see acdfc84.
Note that commit 155e9ab reverted these changes.