Extend the grammar/semantics of `default:` to support boolean expressions

This is expanded version [of this comment](https://github.com/haskell/cabal/issues/3072#issuecomment-174487089):

Currently, the grammar for the `default` field is limited to

```
default:
```

This is perfectly sufficient for properly used automatic flags (i.e. `manual: False` and used in a way which allows the solver to unique determine their value -- there's a separate feature request to have cabal detect "non-functional" uses of automatic flags); however, for manual flags the limitation of allowing only a constant boolean literal in `default:` is rather limiting.

Instead, starting with e.g. `cabal-version:3.0`, we should allow an enhanced version of `default`

```
default:
```

and where `` would be the expression we already support in `if` clauses which currently supports the

- the literals `True/0` and `False/0`,
- binary operator `&&/2`, `||/2`,
- the unary operator `!/1`,
- grouping via `(` and `)`, and the
- builtin predicates:
- `impl/1`
- `flag/1`
- `arch/1`
- `os/1`

Such expression can be statically determined, as the `impl/1`, `arch/1`, and `os/1` predicates are globally constant for a solver session.

The `flag/1` predicate however requires us to impose a limitation though, in order to keep the implementation simple (allow for referential checks during single-pass parsing) and avoid the issue of undecidable flags (e.g. via cyclic references). To that end, the following simple well-formedness rule needs to be added to the specification of `flag/1`

- `flag()` may only refer to a `` which is lexically visible, i.e. *completely* defined before their use-site (i.e. only *after* its `Flag` stanza ends, a flag is considered completely defined; this prefents direct self-referential uses of `flag/1` from within the same Flag stanza)

- Furthermore (as pointed out in https://github.com/haskell/cabal/issues/5328#issuecomment-389450253), a manual flag's default expression must not be refer to an automatic flag.