Code quality improvements

[Steve Kemp]

The last few commits to master were focused on reducing the code-complexity - specifically those reports on the goreportcard:

• https://goreportcard.com/report/github.com/skx/marionette

The site is down at the moment, but the complexity score of >15 is handled via gocyclo. I can use that to confirm I've fixed things.

In addition to the complexity issues I see that some modules aren't tested at 100%, and some not at all.

• Update the parser-test at least since that is achingly close to 100% coverage.
• Add some simple end-to-end tests of file, edit, etc modules.
  • ideally each of our modules should have at least one test-case.
  • but that can wait.

Easy wins should be easy, for example the lexer can be updated to use a lookup table for the majority of its token-types.