Fix lexical scoping

Song is supposed to be lexically scoped, but a bug meant that in many cases it was actually dynamically scoped leading to lots of confusion.

E.g. the following was surprisingly returning 2 rather than throwing an error:

foo() = n
bar(n) = foo()
bar(2)

This release tightens up the lexical scoping rules to behave as you would expect in most functional and C-style languages.