Require rust >= 1.30 and drop libc_core_cvoid conditional

Move the `c_void` re-export to the top-level `lib.rs`.

(apply <#4060> to `main`)
(cherry picked from commit e221887)

Author: joshtriplett

src/fuchsia/mod.rs

@@ -3,6 +3,8 @@
 //! More functions and definitions can be found in the more specific modules
 //! according to the platform in question.
 
+use c_void;
+
 // PUB_TYPE
 
 pub type c_schar = i8;
@@ -4486,5 +4488,3 @@ cfg_if! {
         // Unknown target_arch
     }
 }
-
-pub use ffi::c_void;

src/hermit.rs

@@ -1,5 +1,7 @@
 //! Hermit C type definitions
 
+use c_void;
+
 cfg_if! {
     if #[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))] {
         pub type c_char = u8;
@@ -576,5 +578,3 @@ extern "C" {
     #[link_name = "sys_poll"]
     pub fn poll(fds: *mut pollfd, nfds: nfds_t, timeout: i32) -> i32;
 }
-
-pub use ffi::c_void;

src/lib.rs

@@ -61,6 +61,8 @@ use core::num;
 #[allow(unused_imports)]
 use core::option::Option;
 
+pub use core::ffi::c_void;
+
 cfg_if! {
     if #[cfg(windows)] {
         mod fixed_width_ints;