Refactor benchmark example to reduce stack size

1. Seperate the tests to different hepler functions, according to
   their module
2. Enable ECDSA tests
3. modify README with correct link

Author: Ron Eldor

benchmark/README.md

@@ -6,7 +6,7 @@ This application benchmarks the various cryptographic primitives offered by mbed
 
 Set up your environment if you have not done so already. For instructions, refer to the [main readme](../README.md).
 
-You can also compile this example with the [mbed Online Compiler](https://developer.mbed.org/compiler/) by using [this project](https://developer.mbed.org/teams/mbed-os-examples/code/mbed-os-example-tls-benchmark).
+You can also compile this example with the [mbed Online Compiler](https://os.mbed.com/compiler/) by using [this project](https://developer.mbed.org/teams/mbed-os-examples/code/mbed-os-example-tls-benchmark).
 
 ## Monitoring the application