Fix parse tree for !!

[lionel-]

• Remove one layer of parentheses surrounding bang-bang, e.g. (!! x). This way:

x <- quote(fn)
expr(
  (!! x)(foo)
)

# expands to:
fn(foo)

# instead of:
(fn)(foo)

• What operator precedence do we want for !!? On master it's now binding very tightly, but maybe we should make some exceptions. Maybe operators that are written without space should bing tightlier than !!:

expr(!! x:y)

Currently on master this is equivalent to:

expr((!! x):y)

But maybe this should be:

expr(!! (x:y))

There is a similar issue for ^:

expr(!! x^y)

[hadley]

I think it's most consistent to match the existing infix operators + and -