Test pseudo-namespaces in C

In C you can create pseudo-namespaces by abusing structs of function
pointers, making function calls look like `module.func(x)` instead of
`func(x)`.

A lot of people online spout that this creates overhead, but this should
show that clang>=15.0.7 will still inline such function calls. GCC 11.3.1
is unable  to inline the function, but will still call the function
directly.

dump.asm

@@ -0,0 +1,10 @@
+Dump of assembler code for function main:
+   0x0000000000401130 <+0>:	push   %rax
+   0x0000000000401131 <+1>:	mov    $0x402010,%edi
+   0x0000000000401136 <+6>:	mov    $0x6,%esi
+   0x000000000040113b <+11>:	xor    %eax,%eax
+   0x000000000040113d <+13>:	call   0x401030 <printf@plt>
+   0x0000000000401142 <+18>:	xor    %eax,%eax
+   0x0000000000401144 <+20>:	pop    %rcx
+   0x0000000000401145 <+21>:	ret    
+End of assembler dump.