Extracted type GetVariableValue from ExpressionInterpreter

This is a preliminary step to improving the way the builtin functions
handle types of their arguments.
Handle argument types of builtin functions better
Added nr2char() builtin VimScript function
Plugged nr2char() function into interpreter

+ Allow numbers in the middle of words in the tokenizer
+ Plugged it into the interpreter
Updated naming per pull request comments

Hopefully this will also fix AppVeyor's puzzling refusal to build...
Ugly parse: VimL sometimes allows digits in words
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 #1521 from Kazark/adds-nr2char-function

Adds nr2char() function and refactors builtin function feature
Normalize values from a command line yank

closes #1526
Merge pull request #1527 from Kazark/fixes-list-parsing

Parse, evaluate and echo non-empty VimScript lists