Disable most implicit conversion constructors

We have various classes that have non-explicit constructors that accept
a single argument, which is implicitly making them implicitly
convertible from the argument.  In a few cases, this is desirable (e.g.
implicit conversion of std::string to py::str, or conversion of double
to py::float_); in many others, however, it is unintended (e.g. implicit
conversion of size_t to some pre-declared py::array_t<T> type).

This disables most of the unwanted implicit conversions by marking them
`explicit`, and comments the ones that are deliberately left implicit.