Move Meta.Const submodule into its own module named Leaf.
Begin changing structure of documentation. Add some more tutorials.

Tutorials will be in a sidebar and the pages will just host the API
reference.
Finalise movement of Gears.Meta.Const to Gears.Leaf
Fixes for generic addressof.
Make io::fprint and co. fully stateless. (Issue #26)
Remove silly optimisation and check for numeric value after precision
in io::fprint and co. (Issue #26)
Remove unnecessary checks, add allocator template parameters, and also
switch the + and p verbs for the format-spec. Fixes #26.