Remove to_cstring() naming convention ambiguity

Add a trait for str.strdup() to replace to_cstring() which avoid the
signature ambiguity with .to_string().

Also instruduce CString::yolo() as a shortcut to
CString::new().unwrap() and use it whenever the variable does can be
deallocated by going out of scope.  This is less error prone.

Use some Path.to_c_string() functions where possible.

src/dc_strencode.rs

@@ -1,4 +1,4 @@
-use std::ffi::CStr;
+use std::ffi::{CStr, CString};
 
 use charset::Charset;
 use mmime::mailmime_decode::*;
@@ -710,9 +710,8 @@ unsafe fn print_hex(target: *mut libc::c_char, cur: *const libc::c_char) {
     assert!(!cur.is_null());
 
     let bytes = std::slice::from_raw_parts(cur as *const _, strlen(cur));
-    let raw = to_cstring(format!("={}", &hex::encode_upper(bytes)[..2]));
-    libc::memcpy(target as *mut _, raw as *const _, 4);
-    free(raw as *mut libc::c_void);
+    let raw = CString::yolo(format!("={}", &hex::encode_upper(bytes)[..2]));
+    libc::memcpy(target as *mut _, raw.as_ptr() as *const _, 4);
 }
 
 #[cfg(test)]