Re-Introduce Fn/FnMut/FnOnce as type-erased closures without heap allocs

They are only safe to use as function parameters but will allow more
lightweight (for compile time and binary size) receiving of lambdas,
even with bound members instead of requiring a template every time.

Author: danakj

subspace/CMakeLists.txt

@@ -51,6 +51,7 @@ target_sources(subspace PUBLIC
     "fn/fn_bind.h"
     "fn/fn_defn.h"
     "fn/fn_impl.h"
+    "fn/stackfn_defn.h"
     "iter/__private/into_iterator_archetype.h"
     "iter/__private/iterator_end.h"
     "iter/__private/iterator_loop.h"
@@ -151,6 +152,7 @@ add_executable(subspace_unittests
     "construct/into_unittest.cc"
     "construct/default_unittest.cc"
     "fn/fn_unittest.cc"
+    "fn/stackfn_unittest.cc"
     "iter/iterator_unittest.cc"
     "mem/addressof_unittest.cc"
     "mem/clone_unittest.cc"

subspace/fn/fn.h

@@ -17,3 +17,4 @@
 #include "subspace/fn/fn_bind.h"
 #include "subspace/fn/fn_defn.h"
 #include "subspace/fn/fn_impl.h"
+#include "subspace/fn/stackfn_defn.h"