Single-parameter constructors should be explicit

[kennykerr]

Fixes #413

[oldnewthing]

0-parameter and N-parameter constructors can also be converting constructors. I think we should allow the the 0-parameter converting constructor, and I'm not sure about whether we should allow the N-parameter one.

I'm assuming we are leaving the 1-parameter nullptr conversion constructor enabled.

struct C {
  C();
  C(std::nullptr_t);
  C(Int32);
  C(Int32, Int32);
};

void F(C c);

Statement	Prior to this change	After this change	Desired?
F({});	Allowed	Allowed	Probably
F(nullptr);	Allowed	Allowed(?)	Yes
F(42);	Allowed	Disallowed	Probably not
F({ 42, 96 });	Allowed	Allowed	Unclear
C c;	Allowed	Allowed	Yes
C c{nullptr};	Allowed	Allowed(?)	Yes
C c(42);	Allowed	Allowed	Yes
C c(42, 96);	Allowed	Allowed	Yes
C c = {};	Allowed	Allowed	Probably
C c = nullptr;	Allowed	Allowed(?)	Probably
C c = 42;	Allowed	Disallowed	Probably not
C c = { 42, 96 };	Allowed	Allowed	Unclear

[kennykerr]

@oldnewthing yes, I'm not planning on changing anything about initialization in general and rather leaving the C++ language defaults as is. The nullptr_t is not explicit as doing so would be a bigger breaking change and the value of taking that on is questionable. Having single-parameter constructors explicit solves the chief usability/pit-of-failure issue here.