Improved Container Initialization

[Jason McKesson]

I made some posts a few days ago about adding default initialization in
containers, as well as disambiguating constructors so that you can use
uniform initialization with certain container types (vector<int> {32, 2}
being a confounding example).

Here is a more formal proposal for the idea. It explicitly states every
function that will be added. I made sure to specify things such that no
ABI breakage would occur (ie: new constructors are added, but no
constructor signatures or behavior is changed). It also explains how
unusual constructs (like vector<default_init_t>) will be dealt with.

Feel free to comment on it.

[Nicol Bolas]

Proofreading fail:

This phrase in the Impact on the Standard section:

as well as modify existing ones by adding a default parameter

Ignore it.