make C strings a little more ergonomic

[geofft]

1. See if we can get std::ffi::CString and std::ffi:CStr to work; they're actually in libstd, but I think CString only uses alloc/containers and CStr doesn't even need that. (And decide if we want it?)
2. Find something better than b"foo\0" for compiile-time static C strings. Options include https://github.com/mzabaluev/rust-c-str and https://github.com/abonander/const-cstr (both of which use std::ffi::CStr) or just coding it ourselves as suggested in Consider adding CString/*const i8 literals rust-lang/rfcs#400 (comment) . If we write it ourselves I might argue for calling it c! to keep it easy to read/write.