Extract and document Node#ruby_var_name

It will be used in more places soon.
Compile variable references correctly
Extend high-level specs for variables
Implement Ruby local variable name escaping

YCP variables sometimes start with a capital letter, which is illegal in
Ruby. This commit implements a simple escaping mechnism to deal with
that. The chosen escaping scheme -- prefix all variable names starting
with [A-Z_] by "_" -- shouldn't create any variable name clashes.