Add `:cbrowse` to print imported Cryptol modules

[sauclovian-g]

In the Cryptol repl you can do :browse to print the contents of a module. We ought to be able to do that from the SAWScript repo.

Currently if you use cryptol_load that gives you a CryptolModule that can be printed, but there's no way to do that with an import and we'd rather people used the import syntax. (And we're planning to deprecate cryptol_load and CryptolModule eventually.) Also, we can likely reuse the Cryptol :browse code for a :cbrowse repl command.

An alternative would be to keep CryptolModule and its print code and have import bind CryptolModules; there are some complications with that, though, for selective or partial imports. Also it's messier from a name handling perspective. Unless there's some reason to want to be able to print module contents from inside SAWScript code (vs. a :-command) I think :cbrowse is a better solution.