sys (and bn): make CRYPTO_free() take a *mut c_void insead of a *const c_char

CRYPTO_free() ends up being used for a variety of types of data, not just
c_char. And it essentially takes full ownership of the type, making
*mut appropriate.

With this change it also more closely (exactly) matches the C defintion:
	void CRYPTO_free(void *ptr);

openssl-sys/src/lib.rs

@@ -295,7 +295,7 @@ extern "C" {
                                                            n: c_int,
                                                            file: *const c_char,
                                                            line: c_int));
-    pub fn CRYPTO_free(buf: *const c_char);
+    pub fn CRYPTO_free(buf: *mut c_void);
     pub fn CRYPTO_memcmp(a: *const c_void, b: *const c_void,
                          len: size_t) -> c_int;
 

src/bn/mod.rs

@@ -1,4 +1,4 @@
-use libc::{c_int, c_ulong};
+use libc::{c_int, c_ulong, c_void};
 use std::{fmt, ptr};
 use std::c_str::CString;
 
@@ -348,7 +348,7 @@ impl BigNum {
             assert!(!buf.is_null());
             let c_str = CString::new(buf, false);
             let str = c_str.as_str().unwrap().to_string();
-            ffi::CRYPTO_free(buf);
+            ffi::CRYPTO_free(buf as *mut c_void);
             str
         }
     }