Add support for configuring assert behavior

[np-c0mp1ete]

Hello,

it seems currently there's no way to replace an assert macro included from assert.h with a custom assert behavior.
assert is used for instance when accessing a value stored in an expected object:

constexpr value_type const * operator ->() const
{
    return assert( has_value() ), contained.value_ptr();
}

It'd be nice to be able to configure assert behavior.

Best regards,
Nikolai