Revise the declaring/primary constructor proposal

[eernstg]

This PR changes the declaring/primary constructor proposal to use a different scoping structure, to clarify the placement of the modifier const with a declaring constructor, to mention the principle that the class header should contain all parts of the signature of a declaring constructor if it contains any of them (e.g., class const C(); makes the primary constructor constant, but we can't do class C() { const this; }). Moreover, this PR includes several other minor clarifications and corrections.

[munificent]

Very nice! I agree the scoping is unusual. But it's consistent with initializer lists and those seem to have done the right thing in practice for years without user complaint.

[eernstg]

Thanks for great reviews! More changes are coming tomorrow.