ffi: `const char*` parameters have no specified length (null-terminated)

I come from another language with slices, so it's rather vexing that quiche's FFI doesn't expose string parameters as slices (pointer + length), but rather as null-terminated arrays (pointer + memory leak). This means that before sending a string slice to quiche I have to null-terminate it and pass only its pointer to quiche, only for quiche to find the null-terminator and re-slice it anyway. In other words:

> safe bounds-checked slice ⇒ unsafe null-terminated string ⇒ safe bounds-checked slice

I would prefer if there were alternative functions that take a pointer & length so that I don't have to do this dance. Many C libraries (including portions of the C standard library) also expect `const char*`s to be passed alongside a length, so I don't think it's unreasonable to expect it here.