Add incomplete_allocator tests to seq containers.
Ensure vector<Incomplete> and list<Incomplete>.
Support forward_list<Incomplete>.

Get the begin node type (which does not require the completeness of
value_type) without looking at data node type's definition.
Support deque<Incomplete>.

Discard the _BlockSize template argument from __deque_iterator and
calculate the block size with a (optimized out) function.

This change BREAKS ABI.

Warning: Do not try supporting deque<Incomplete> without breaking
the ABI; ODR violation is in front of you.
Ensure container adaptors' completeness support.
Ensure assoc containers' completeness support.
Ensure unordered containers' completeness support.
Test the Container completeness requirements.