Properly support prefix operator precedence printing

The pretty printer is supposed to produce a minimal-parentheses
printing for expressions. However, for prefix operators, we were

Properly support prefix operator precedence printing

The pretty printer is supposed to produce a minimal-parentheses
printing for expressions. However, for prefix operators, we were
failing to do so in ways that are practically meaningful, e.g.
$a = yield from $b produced redundant parentheses around the
yield from.

For prefix operators, the precedence of the direct parent operator
is not actually relevant: What matters is the precedence of any
infix operator whose LHS it appears on. For example, $a . include $b
does not require parentheses, but (include $a) . $b does.

To handle this, keep separate track of the parent operator
precedence, and a special LHS precedence which is used for unary
operator printing only. Because the LHS precedence may not come
from the direct parent, we have to pass it down the call stack.
And if we do that, we may as well do the same for the parent
precedence.

This also fixes an integration test failure with php-src, because
arrow function precedence was previously not respected (and would
be ugly to respect without this change).

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Split up pretty printer test in stmt/expr

The list was getting unweildly.

Also improve error message when parsing fails in pretty printer
test and extend some tests.