Improve the parser to match the spec more closely

[jaseemabid]

A short list of programs that should parse but wont at the moment.

• Functions without args (f)
• Simple letrec.

(letrec ((e (lambda (x) (if (zero? x) #t (o (dec x)))))
         (o (lambda (x) (if (zero? x) #f (e (dec x))))))
  (e 25))

[vigneshsarma]

We probably close this,

1. works now
2. Error looks like

thread 'main' panicked at 'Failed to parse input program `(inc 42`: Error { message: "Error((\"\", Char))" }', src/libcore/result.rs:999:5