Uminus improvements.

[toots]

This PR improves unitary minus by adding support for any - <expr> via a top-level precedence. This fixes #3405

The other part of these changes add } to our manual unitary minus hack. This hack is because our grammar:

• Ignores new lines
• Supports <expr> - <expr> and - <expr>

This means that, the following expressions are the same:

x
-123

and:

x - 123

However, what we really have been doing since the beginning is treating line return as an expression separator. This is the same reason we differentiate between:

def f(x, y)
  true
 end

and:

def f
  (x,y )
  true
end

My recommendation for future improvements would be:

• Find a way to bring back line return as expression separator
• Force all def functions to use =

BTW, we also support ; for expression separator.. 🙂