Update CakeML tutorial to use monadic translator

[myreen]

Currently the CakeML tutorial, i.e. the files under tutorial/solutions and particularly wordfreqProgScript.sml, is based on CF proofs.

Since the monadic translator, which is described in Proof-Producing Synthesis of CakeML with I/O and Local State from Monadic HOL Functions, could produce the same program in a nicer way, I think the tutorial should be ported to use the monadic translator.

One can take inspiration from existing examples that use the monadic translator.

[myreen]

This is still relevant. We should use the monadic translator much more.