true/false are not reserved words

if you don't believe me, try this in nix-repl:

`let false = true; in false`
trifecta: reserved(Op) shouldn't return a value

this fixes another difference between the trifecta / parsec interface.
antiquotes + improve pretty printer + restructure

This commit improves the pretty printing and adds support for
antiquotes. It also fixes an issue with the parser that caused `[if true
then false else true]` to parse successfully, even though that is not a
valid nix expression.

The pretty printer now produces a lot more readable output and also
supports operator precedences.

The changes to the AST are:

  * strings are no longer atomic, because they may contain other
  expressions in the form of antiquotes. For strings, the new type
  NString is introduced and the constructor NStr is added to NExprF
  * the unused NVar constructor of NExprF is removed
  * operators are now represented uniformly so that the pretty printer
  can lookup information about operators (in particular, associativity
  and precedence)
  * the NArgs constructor is removed. The first argument of the NAbs
  constructor now directly represents the lambda arguments.
  * the select and the hasattr operator are moved into NExpr because
  they are special (they only accept a selector as second argument, and
  select also supports 'or')

The list of operators is now in Types.hs and Parser.hs re-uses that list
to build the parser. This is required because the pretty printer and
parser both need access to operator precedences.

Parser and evaluator also support dynamic attributes and attributes with
dots now. As an example, `let b.a = 3; b.c = { e = {}; }; b.c.e.${"f"} =
4; in b` is parsed and evaluated correctly. As a side effect, NSym
values now don't evaluate to themselves anymore, but instead to the
value retrieved by looking up the variable in the current environment.

Support for evaluating `inherit` bindings was removed because it was
broken before (`{ inherit a; }` would evaluate to a set where the
attribute `a` had the value `NSym a`, not the value of `a`).

The manual Show instances for the AST were replaced by derived
ones, because the manual instances often resulted in output were it was
difficult to determine the missing parentheses.
parser: add tests and fix bugs
fix pretty printing of argument sets
fix error parsing paths
fix error parsing `//`  (opLetter characters)
now really fix parsing paths
More tests for select expressions
Merge pull request #13 from bennofs/restructure

antiquotes + improve pretty printer + restructure