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
Merge pull request #1521 from Kazark/adds-nr2char-function

Adds nr2char() function and refactors builtin function feature