ToField for negative numbers should be parenthesized

[ChickenProp]

Postgresql parses - <num> :: <type> as - (<num> :: <type>). That can cause problems when trying to reference a type's minimum value:

> select -2147483648 :: int4;
ERROR:  integer out of range
> select (-2147483648) :: int4;
    int4     
-------------
 -2147483648

In the context of postgresql-simple, we can get problems if we try to do e.g.

execute conn "INSERT INTO foo (col) VALUES (?::int4)" (Only someInt)
query conn "SELECT * FROM ?" (Values ["int4"] [Only someInt])

I think postgres has symmetric ranges for non-integer numbers, but since those can get cast to ints, it can still make a difference whether they're parenthesized:

> select -2147483648.2 :: int4;
ERROR:  integer out of range
> select (-2147483648.2) :: int4;
    int4     
-------------
 -2147483648
(1 row)