The Fn*Ref types don't use uintptr_t afterall, they use a union

This allows them to be constexpr constructed from a lambda, even if
they can't be constexpr-evaluated.

Author: danakj

subspace/fn/fn_ref.h

@@ -14,8 +14,6 @@
 
 #pragma once
 
-#include <stdint.h>
-
 #include "subspace/fn/callable.h"
 #include "subspace/macros/lifetimebound.h"
 #include "subspace/mem/addressof.h"

subspace/lib/lib.cc

@@ -26,12 +26,6 @@ static_assert(INT_MAX == INT32_MAX);
 static_assert(sus::mem::size_of<size_t>() >= 4);
 static_assert(sus::mem::size_of<size_t>() <= 8);
 
-// FnRef types expect to be able to store function and object pointers in a
-// uintptr_t.
-static_assert(sus::mem::size_of<void*>() <= sus::mem::size_of<uintptr_t>());
-static_assert(sus::mem::size_of<void (*)()>() <=
-              sus::mem::size_of<uintptr_t>());
-
 // TODO: Consider if we should only support little endian? We probably make this
 // assumption. Support for endian *conversion* is still important for network
 // byte order etc.