Add explicit keyword for generated constructor

[makotokato]

https://bugzilla.mozilla.org/show_bug.cgi?id=1868324

I would like to add explicit keyword to generated C++ constructor.

[robertbastian]

Is this a breaking change for existing clients?

[makotokato]

Our coding style needs explicit keyword for non copy/move constructor. So we have to modify it to use icu_capi since current FFI API doesn't accept our rule. explicit keyword can prevent unexpected conversion.

Also, see #349 for diplomat_runtime.hpp.

[robertbastian]

I understand the problem. My question is, can this change break compilation for anyone who is using ICU4X today?

[robertbastian]

I guess it can if someone is using implicit conversion today.

[Manishearth]

It depends on whether you consider the raw pointer API to be actually public. I don't but we should talk about it.

[makotokato]

I don't think adding explicit keyword breaks existed code. But feel free to reject this PR.

[Manishearth]

It is for anyone using the constructor implicitly. I think that's not a big deal though, because that constructor is a major footgun.

[Manishearth]

@sffc mind checking and fixing this in CPP2?