Handle the new clippy::manual_c_str_literals lint

[nyurik]

Clippy 1.83 introduced manual_c_str_literals on this type of generated code:

#[allow(unsafe_code)]
pub const VSL_CLASS: &::std::ffi::CStr =
    unsafe { ::std::ffi::CStr::from_bytes_with_nul_unchecked(b"Log\0") };

warning: calling `CStr::new` with a byte string literal
   --> .../out/bindings.rs:708:14
    |
708 |     unsafe { ::std::ffi::CStr::from_bytes_with_nul_unchecked(b"Log\0") };
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use a `c""` literal: `c"Log"`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_c_str_literals
    = note: `#[warn(clippy::manual_c_str_literals)]` on by default

Possible solutions

• introduce a new generate_cstr_literal(bool) flag
• change generate_cstr(bool) to take an enum instead, e.g. Off, CStrObjects, CStrLiterals
• add #[allow(clippy::manual_c_str_literals)] to each instance of calling from_bytes_with_nul_unchecked