Started a suite of parser tests of lists
Parse lists other than just empty lists
Plug list expressions into expression interpreter

This repairs some tests, which I had correctly guessed would be broken
by the last commit
Support :echo for non-empty lists
Refine :echo ["foo"] - quote strings when echoed
Merge pull request #1527 from Kazark/fixes-list-parsing

Parse, evaluate and echo non-empty VimScript lists