added useful string conversions in rust

sheets/_rust/strings

@@ -0,0 +1,45 @@
+let s: String = ...
+let st: &str = ...
+let u: &[u8] = ...
+let b: [u8; 3] = b"foo"
+let v: Vec<u8> = ...
+let os: OsString = ...
+let ost: OsStr = ...
+
+From       To         Use                                    Comment
+----       --         ---                                    -------
+&str     -> String    String::from(st)
+&str     -> &[u8]     st.as_bytes()
+&str     -> Vec<u8>   st.as_bytes().to_owned()               via &[u8]
+&str     -> &OsStr    OsStr::new(st)
+
+String   -> &str      &s                                     alt. s.as_str()
+String   -> &[u8]     s.as_bytes()
+String   -> Vec<u8>   s.into_bytes()
+String   -> OsString  OsString::from(s)
+
+&[u8]    -> &str      str::from_utf8(u).unwrap()
+&[u8]    -> String    String::from_utf8(u).unwrap()
+&[u8]    -> Vec<u8>   u.to_owned()
+&[u8]    -> &OsStr    OsStr::from_bytes(u)                   use std::os::unix::ffi::OsStrExt;
+
+[u8; 3]  -> &[u8]     &b[..]                                 byte literal
+[u8; 3]  -> &[u8]     "foo".as_bytes()                       alternative via utf8 literal
+
+Vec<u8>  -> &str      str::from_utf8(&v).unwrap()            via &[u8]
+Vec<u8>  -> String    String::from_utf8(v)
+Vec<u8>  -> &[u8]     &v
+Vec<u8>  -> OsString  OsString::from_vec(v)                  use std::os::unix::ffi::OsStringExt;
+
+&OsStr   -> &str      ost.to_str().unwrap()
+&OsStr   -> String    ost.to_os_string().into_string()       via OsString
+                         .unwrap()
+&OsStr   -> Cow<str>  ost.to_string_lossy()                  Unicode replacement characters
+&OsStr   -> OsString  ost.to_os_string()
+&OsStr   -> &[u8]     ost.as_bytes()                         use std::os::unix::ffi::OsStringExt;
+
+OsString -> String    os.into_string().unwrap()              returns original OsString on failure
+OsString -> &str      os.to_str().unwrap()
+OsString -> &OsStr    os.as_os_str()
+OsString -> Vec<u8>   os.into_vec()                          use std::os::unix::ffi::OsStringExt;
+