Add find_or_try_insert_with for fallible insert functions

[krisvanrens]

From the examples:

#include <cassert>
#include <memo_cache.hpp>

mc::memo_cache<int, std::string, 4> c;

assert(!c.contains(42));

auto v1 = c.find_or_try_insert_with(42, [] -> std::expected<std::string, std::string> { return "The Answer"; });

assert(v1 == "The Answer");
assert(c.find(42).has_value());
assert(c.find(42).value() == "The Answer");

auto v2 = c.find_or_try_insert_with(42, [] -> std::expected<std::string, std::string> { return std::unexpected{"Dunno"}; });

assert(!v2.has_value());
assert(v2.error() == "Dunno");

But this doesn't work yet because the compiler is not able to deduce fallible result type. See also: https://www.godbolt.org/z/o93qGsvzY