New features: Enable warnings/hints, describe definitions

1. A short description about the parsed definitions will now be displayed:

frege> fib = 0 : 1 : zipWith (+) fib fib.tail
value fib :: [Int]

frege> f x = x + x
function f :: Num α => α -> α

frege> :{
data YesNo = Yes | No
derive Show YesNo
:}

data type YesNo :: *
instance Show  YesNo

frege> 

2. Warnings and hints are enabled:

frege> g x = g x
function g :: α -> β
[WARNING]: application of g will diverge.

frege> bar x = x + x where baz = 10
function bar :: Num α => α -> α
[WARNING]: application of g will diverge.
[HINT]: value `baz` is not used anywhere.

frege>