Add `explicit` to all one-argument ctors

[agcapps]

Survey the code: find all the single-argument constructors where we should add the explicit keyword. There may be places where we want to allow implicit construction. We might not be able to add explicit to initializer list constructors.

[rhornung67]

Please add cases in the code that you see and should be discussed here:

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[agcapps]

Done for now. Will reopen if this bites us again.