Stop creating a special node for interpolation

Just build AST with synthesized to_string()

"a#{b}c"
'a' + b.to_string() + 'c'

Conflicts:
	lib/rip/nodes/interpolation.rb
Parse interpolation in regular expression as regular expression concatenation

/a#{b}c/
/a/ + b.to_string().to_regular_expression() + /c/
No special nodes for interpolation