Functions: introduce multi-function namespace

This moves all multi-function related code in the `functions` module
into a new `multi_function` namespace. This is similar to how there
is a `lazy_function` namespace.

The main benefit of this is that many types names that were prefixed
with `MF` (for "multi function") can be simplified.

There is also a common shorthand for the `multi_function` namespace: `mf`.
This is also similar to lazy-functions where the shortened namespace
is called `lf`.

source/blender/functions/intern/multi_function_params.cc

@@ -2,7 +2,7 @@
 
 #include "FN_multi_function_params.hh"
 
-namespace blender::fn {
+namespace blender::fn::multi_function {
 
 GMutableSpan MFParams::ensure_dummy_single_output(int param_index)
 {
@@ -27,4 +27,4 @@ GMutableSpan MFParams::ensure_dummy_single_output(int param_index)
   return span;
 }
 
-}  // namespace blender::fn
+}  // namespace blender::fn::multi_function