[strings] Use "Equivalent to" where appropriate

[sdutoit]

From an email by Jeffrey Yasskin:

Many functions in the strings clause use formulations like:

[string.cons]/15:

Effects: Same as basic_string(il.begin(), il.end(), a).

[string.capacity]/15:

Effects: Behaves as if the function calls:
erase(begin(), end());

[string::insert]/27:

Effects: insert(p, il.begin(), il.end()).
Returns: [what the above insert() call returns]

instead of the "Equivalent to" wording defined in
[structure.specifications]/4. These should all be changed to
"Equivalent to" for consistency and conciseness.

[burblebee]

This needs to be opened as an LWG issue. I've asked for an issue to be opened for this throughout the spec, but I don't know it's number yet.

[jensmaurer]

LWG 2679, but that doesn't cover the places mentioned above.

[jensmaurer]

I've asked for another LWG issue, explicitly listing the places that need fixing.

[jensmaurer]

LWG 2841