Translate source AST types before `Repl` module

This issue is about moving the translation of `dec` from compiler/bootstrap/translation

https://github.com/CakeML/cakeml/blob/master/compiler/bootstrap/translation/decProgScript.sml

to its own `Ast` module right before the definition of the `Repl` module

https://github.com/CakeML/cakeml/blob/master/compiler/repl/repl_moduleProgScript.sml

Note that this change makes the source AST types available to programmers in the REPL. The automatically generated pretty printing functions should also be installed in the new `Ast` module, so that users get pretty printing for the AST types.

This issue needs to be done before #1314.

Start with compiler/bootstrap/translation/decProgScript.sml to trace the translation of dec, then inspect compiler/repl/repl_moduleProgScript.sml at the definition of the Repl module. Move that translation into a new Ast module before Repl and install the automatically generated pretty-printing functions there, so source AST types and their pretty printing are available to REPL programmers; complete this before #1314.

Written by the indexing model from the issue text.