Update utility component documentation to mention constexpr versions of move and forward
Close #12

core::invoke is now usable with constexpr values. PLEASE NOTE: GCC 4.8.1
currently has a bug with function pointers losing their constexpr-ness
when passed to a constexpr function. This is completely out of my
control. However, it also severely limits the usage of core::invoke in
some situations, as frustrating as it may be.
Update utility component 'value_at<N>' to work correctly.

It would fail when recursing, and this was due to an incomplete proposal
from N3761.

Also moved the constexpr versions of forward and move higher up in the
file for use later on.