[SUGGESTION] Add inspection of std::variant and std::optional values

[filipsajdak]

Current implementation of inspect and is allows to inspect types. #79 add inspection capabilities to check values of variables (what allows to replace cpp1 switches).

This change introduce possibility to inspect values stored in std::variant and std::optional what make given code work:

v : std::variant<lexeme, lexeme2, std::string, int, double, char, my_struct> = lexeme2::hash;

std::cout << inspect v -> std::string {
    is lexeme::hash  = "original lexeme";
    is lexeme2::hash = "fake lexeme2";
    is "hash"        = "it is string 'hash'";
    is 42            = "the answer is 42";
    is 1.23          = "the answer is 1.23";
    is 'a'           = "this is 'a'";
    is _             = "don't know";
} << std::endl;

or you can use it as if statement with std::optional

o : std::optional<std::string> = "hash";

if o is "hash" {
    std::cout << "Optional has 'hash' value" << std::endl;
} else {
    std::cout << "There is something else" << std::endl;
}

or std::variant

if v is 1.23 {
  std::cout << "This is 1.23" << std::end;
}

This PR is based on #79

[filipsajdak]

Rebase to main. Make is() functions constexpr

[filipsajdak]

Might require #90 for some use cases.

[hsutter]

Might require #90 for some use cases.

Should these be merged? Should all the is/as PRs be merged (also #79, #106, #108)? (Not suggesting, just asking. As I look through them I'm trying to grok the overlap.)

[fecjanky]

Conceptually can we make std::optional<T> to be isomorphic to std::variant<std::monostate,T> from the cpp2 language/library point of view as these types are isomorphic mathematically as well? This would definitely simplify things on multiple levels.

[hsutter]

Thanks! About half of this (parsing and literals) seems to be duplicated from #79, so I'll focus on the variant, optional, and any parts... more soon...

[hsutter]

OK, half of this which wasn't about variant/optional/any was merged in the commit c77de70 that closed #79, and I'm going to apply the rest for those three standard types in a separate commit to follow the pattern of commit c77de70 to use the two-parameter "is value" pattern.

Thanks again!

[hsutter]

I've now added this with 2f37277 - thanks again.