[Foldable] Rename the different fold variants

- Rename fold{l,r,l1,r1} to fold.{right,left} with overloads.
- Rename foldlM/foldrM to monadic_fold.{left,right}
- Add no-state variants of the monadic folds
- Improve the fold's documentation
- Deprecate the previous folds; they'll be removed soon

Fixes #18.

example/string.cpp

@@ -59,7 +59,7 @@ BOOST_HANA_CONSTANT_CHECK(
 
 //! [foldable]
 auto sum_string = [](auto str) {
-    return foldl(str, int_<0>, [](auto sum, auto c) {
+    return fold.left(str, int_<0>, [](auto sum, auto c) {
         constexpr int i = value(c) - 48; // convert character to decimal
         return sum + int_<i>;
     });