[common] add BinarySearch module and Stringfy (#7294)

This commit adds set of generic binary search related functions in
OpenThread core under `BinarySearch` module. The binary search module
replaces and enhances the existing `Lookup` table module (which was
limited to search based on strings and mainly used by CLI to map from
command string to a handler function).

This commit also adds a new class `Stringfy` in `string.hpp` to help
convert from a set of `uint16_t` values (e.g., a non-sequential
`enum`) to strings using binary search in a table. The new methods are
used for converting `Coap::Code` and `IpProto` enumerations to
corresponding strings.

src/cli/cli_udp.cpp

@@ -277,7 +277,7 @@ otError UdpExample::Process(Arg aArgs[])
         ExitNow();
     }
 
-    command = Utils::LookupTable::Find(aArgs[0].GetCString(), sCommands);
+    command = BinarySearch::Find(aArgs[0].GetCString(), sCommands);
     VerifyOrExit(command != nullptr, error = OT_ERROR_INVALID_COMMAND);
 
     error = (this->*command->mHandler)(aArgs + 1);