Support changing the error type via `.or_else`

[Quuxplusone]

Today I commented on P2505R0 that it should be designed to permit this code:

std::expected<Connection, PortableError> convertError(SSLError e) {
    return std::unexpected(PortableError(e));
}

std::expected<Connection, SSLError> e1 = ~~~;
std::expected<Connection, PortableError> e2 = e1.or_else(convertError);

This doesn't work with tl::expected today. I plan to submit a PR for this feature, but will wait for some of my preliminary PRs to get merged first.

[azais-corentin]

As a workaround, this is exactly what .map_error does.

Example:

tl::expected<Connection, PortableError> translateError(SSLError e) {
    return tl::make_unexpected(PortableError(e));
}

tl::expected<Connection, PortableError> s = e1.map_error(translateError);