nb::handle: make bool conversion explicit to avoid ambiguity with integer conversions in derived types

[oremanj]

An explicit operator bool() can still be used in conditional tests without a cast (if (h) { ... }) and is a better match for the explicitness of, for example, operator int() in nb::int_. Without this change, if you write

void fn(nb::int_ ival) {
    int i = ival;
    printf("%d\n", i);
}

then fn(nb::int_(42)) will print 1. (Only implicit conversions are allowed by the initialization style used, so the implicit operator bool in nb::handle is eligible while the explicit operator int in nb::int_ is not.)

[wjakob]

Oh yes, let's have this. I am surprised it was not explicit to start with.