[unum] Implement the remaining methods from unum.h

Not very swell with test coverage now, since most remaining methods are
getters and setters.

Some attempts at generalizing the getters and setters through macros,
with unremarkable results.

Issue google#141

rust_icu_common/src/lib.rs

@@ -146,6 +146,12 @@ impl From<std::string::FromUtf8Error> for Error {
     }
 }
 
+impl From<anyhow::Error> for Error {
+    fn from(e: anyhow::Error) -> Self {
+        Self::wrapper(e)
+    }
+}
+
 impl Into<std::fmt::Error> for Error {
     fn into(self) -> std::fmt::Error {
         // It is not possible to transfer any info into std::fmt::Error, so we log instead.