It looks like everyone that uses libddprof has a variant of this
macro, so I think it makes sense to just have it in `ffi.h`.

This specific implementation is built to only work with string
literals, aka there's no accidental/unexpected  `strlen` going on.

This macro relies on a weird trick to fail compilation when you pass
in a char* and not a literal (the trick is that
`char *foo = "" "foo";` is valid but `char *bar; char *foo = "" bar;`
is not), but the error message would be kinda cryptic.

Since most compilers show the macro when the compilation fails,
I've added a nice comment which gets shown by compilers:

```
foo.c: In function 'endpoint_from':
foo.c:133:27: error: expected '}' before 's'
   byte_slice_from_literal(s);
                           ^
foo.c:15:113: note: in definition of macro 'byte_slice_from_literal'
   /* NOTE: Compilation fails if you pass in a char* instead of a literal */ ((ddprof_ffi_CharSlice) {.ptr = "" string, .len = sizeof(string) - 1})
                                                                                                                ^~~~~~
foo.c:15:91: note: to match this '{'
   /* NOTE: Compilation fails if you pass in a char* instead of a literal */ ((ddprof_ffi_CharSlice) {.ptr = "" string, .len = sizeof(string) - 1})
```