Deprecate std.metastrings.

std.metastrings is no longer required, because the corresponding
normal functions work in CTFE, and std.metastrings is actually very
inefficient.
Added CTFE-ability tests to some stuff in std.conv and std.string.

These tests are for the functionality that std.metastrings provides, and
we need to ensure that the normal functions which are replacing
those in std.metastrings continue to work in CTFE.